If and EMP attack happens on the US, society will collapse shortly from a permanent loss of electricity and all the domino effects of that. So, who will be the groups of people that have most of the power from the time of the event, through the collapse, and many years beyond?
FIRST 2 WEEKS POST COLLAPSE
WHO'S IN POWER?: THE GOVERNMENT
If it is an EMP attack, most people might not understand what is happening and they will wonder why it's taking so long for the power company to get the electricity back on. There will still be a rule of law to the extent that total anarchy will not reign in the first 2 weeks. The power will still be in the hands of police, military, and government, only in the sense that there aren't any rival power groups that will overcome these power structures. However, with each passing day in the first 2 weeks, the mobs of rioting people will overcome any orders given by the government. With lack of communication and mobility and the fact that food is running out, the government, police, and military will begin to collapse. Some military bases may maintain power, depending on their location, supplies they have on hand, and the actions taken by the officers. Rioting and anarchy will begin in some places quicker than others. It might be possible that towns with very civil people remain calm for a long time into the collapse, before their society is disrupted. You will want to bug out in this phase, before gangs and road blocks form.
2 WEEKS - 6 WEEKS POST COLLAPSE
WHO'S IN POWER?: SMALL TIME THIEVES
During this period, the stores have no more food and the police and National Guard have given up, so they could be with and protect their families. The collapse will happen at different speeds for different places, so there may be some areas where the police/national guard/military still have control. During this period, people are looking for more food and perhaps clean water. The power belongs to the individuals who are willing to be violent to get what they want. The power will be in the hands of individual and small groups of criminals. This is also the phase where people will start migrating for clean water and food. People will be looking for food warehouses, farms, and places to fish and hunt. Because of all the migration and camping out near bodies of water, it will be a very dangerous and chaotic time. There will be many murders for scraps of food and many girls will be raped. The President and all the important Washington suits will be in their bunkers. They will think they still have continuity of government and will be trying to give orders, but in reality, they've lost control and they may never be back in control. However, if it is an EMP attack and only North America has been attacked, there will be an effort, which will probably be successful, to escort the President and others to the UK for the time being. The United States government will essentially be a government in exile.
6 WEEKS - 1.5 YEARS
WHO'S IN POWER?: CITY WARLORDS, CARTELS, GANGS
After the initial couple phases, the bad guys will learn really quickly that whoever can organize their crime, will rise to power. Criminals and street gangs will switch from a mentality of stealing when they are hungry to stealing 24/7 and stockpiling as much supplies as possible, while recruiting as many gangsters as possible. It will become organized. They will send out teams of 5- 10 guys in trucks all armed with weapons. They will systematically sweep neighborhoods and hit promising areas to stockpile everything of value. At first, these groups will be street gangs. Then, when prisoners are let out, prison gangs can rise to power too. Then, when the border patrol loses control of the border (as if they even have control now), the full strength of the drug cartels will come flying into the southwest US. These guys are more dangerous than the regular street gangs and prison gangs of the US, because they are accustomed to killing and are very smart when it comes to organizing their crime. In fact, they are so good at organizing their crime that they will probably take a bit of a hit when they lose communication, but they will soon overcome those shortcomings and become the strongest groups in places like San Diego, Los Angeles, Phoenix, El Paso, San Antonio, and Houston. After 1 year, these guys will be solidly in place as the strongest groups in America. They will grow their numbers quickly as they promise new recruits food. The good guys will still outnumber the bad guys, but the good guys will be spread out on their homesteads across rural America. After a year, stockpiles will begin to dwindle and gasoline will degrade. The warlords will realize that if they are to maintain their power, they will need to shift strategies. At first, they will go further and further for food raids, but anybody attempting to farm near a city, will realize it is too dangerous, because the gangs will likely come raid them. So, all the farmers will move further from the cities or hide their farms better, like in the mountains away from roads. The gangs will need to do one of two things: 1) move to farmland with fresh water and begin their own farming operation or 2) acquire crude oil and refine it to allow them to continue their food raids, which will need to become multiple day expeditions. I imagine most groups won't have the skill or ability to acquire and refine their fuel, so they will switch to farmland menaces.
1.5 YEARS - 6 YEARS
WHO'S IN POWER?: FEUDAL DICTATORS & MILITARY SETTLEMENTS
From the period of 6 weeks to 6 years, there will be competition for resources among rival gangs, which can actually be good. Our hope is that they fight each other so much that they weaken themselves to the point they aren't a threat to the more peaceful people who are trying to rebuild.
Meanwhile, if the scenario is an EMP attack on America, I believe that our military abroad and our allies will make an attempt to rescue Americans. They will do this in 3 ways: 1) They will make food supply drops. 2) They will muster as many ships as possible to save people and bring them to other countries as refugees. 3) They will attempt to carve out safe areas and get our infrastructure going again. They will bring new transformers and attempt to plug them in and get facilities working again. They will bring plenty of engineers and electricians and doctors, etc with them. This will be a really tough job, but I think they will be partially successful if they do it right. Hopefully, they won't attempt this on large cities, but small coastal cities, where they have a better chance of succeeding. Once they get a foothold there, they can begin to expand outward and get more areas up and running, but it will be a major challenge as a lot of facilities have been burned to the ground. Also, the collapse of America will result in a global economic crash, so securing aid for the 400 million people living inside the United States will be very difficult. If the military is capable, they should utilize their assets to do strikes on the cartels, gangs, etc. The mere existence of those bad guys will spell disaster for so many and will delay the recovery of America. So if there are any capable helicopters, jets, guns, and mortars, we should strike them hard, but only the worst, most ruthless ones.
6 YEARS - 30 YEARS
WHO'S IN POWER?: THE GOOD GUYS
All the while the warlords, cartels, and gangs have been controlling the cities and lowlands, the preppers, survivalists, and other 'good guys' have been surviving on their own settlements in rural areas and mountains that were unaffected by the hegemony of the 'bad guy' power structures. At first, the good guys will be individual families and small groups surviving on their own, but as the years go by, they will develop trade networks, alliances, and some will come together and centralize their power around major trade centers that also have plentiful fresh water, arable land, and the ability to produce electricity. Most of these powerful trade centers will rise up from the new settlements in the mountains and rural areas. Some will be lucky enough to develop from small towns that survived any outside attack and avoided starvation. The same principles that made America great will make these good guy cities more powerful than the once powerful cartels, gangs, warlords, and dictators. While the bad guys utilize forced labor, fixed wages, monarchical land ownership, injustices, and a lack of moral values, the good guys will utilize the equality of humans, free market trade, private land ownership, and solid moral values, which will allow the good guys to rise to power once again. The corrupt ways of the bad guys won't be able to stand forever. It may not be exactly 6 years after the collapse, but at some point, the good guys will become more powerful. Around that time, there will be wars between the good guys and the bad guys. The good guys from the mountains will be at war with the bad guys of the valleys (which interestingly enough has been a theme of human warfare throughout history--highlanders vs lowlanders). The most powerful good guy city-states will probably be in the mountains of Utah, because of the presence of the security of the mountains, the prepper mentality of many Mormons, and the moral values of the Mormons and Christians. Being highly structured, the Mormon Church itself might have a big role in organizing the survivors and strengthening them. You will also find powerful good guy city-states in most of the red states.
Paralleling the good guys in the mountains and rural areas, the military settlements are 'good guys' as well. Assuming they have any luck, they might be able to bring back to life some seaside cities, but like I said, it will be hard. Most cities don't have their own power plant, many buildings will have burned down, the bad guys might be in charge once the foreigners arrive, a huge amount of equipment will have to be imported, and a security perimeter will need to be maintained all the time. Also, when America collapses, it will cause a worldwide economic collapse, which will diminish foreign aid efforts by other countries.
WHO'S IN POWER?: GOOD GUY KINGDOM STATES OF AMERICA
Looking 30 plus years into my crystal ball, I envision that the good guy city-states will expand and bring stability to more areas, which will develop into regional kingdoms. Most of these kingdoms will be democracies and the leaders will still remember a great nation that was called USA. They will desire to reunite and rebirth the United States, but it won't quite be like it used to be. Eventually, we might get the name back 'United States of America', but the states will be based on the newly formed kingdoms instead of the former borders. They will have strange borders that are based on whatever areas were conquered rather than nice straight lines of latitude and longitude. If there are any bad guy dictatorships left over, they will be defeated in battle or revolutions will happen from within, which will turn them into democracies. During this phase, things will become more stable and there will be less fighting. With the increased stability, international trade will resume, which will bring back manufactured goods and technology to get America back on track. However, as a result of the collapse, we will essentially be a third world nation, so there won't be a lot of money to buy goods from the international suppliers. It will be a long haul, but I suppose America can be back on top of the world's stage in 80 to 150 years, but by that time, technology and the global economy might be so advanced that Americans find themselves in a severe disadvantage.
It will not always be the mountain/rural good guys vs the gang/warlord bad guys. There will be other groups that find themselves in power as well. For example, some groups will form which only do the minimum criminal activity necessary to survive. Their preference will be to do things the honest way, but they place their survival and security above all else. They will be a 'gray area group.' You might also have some military guys or police officers that go bad and play by their own rules, but they are much better than the cartels. You might also have some green living co-ops that actually flourish well after the collapse.
So there you have it--a shift in power from the modern authorities, to the bad guys, and back to the good guys. I hope the power shifts happen this quickly. I used to think the power shift would take much longer, but the primary issues, which force a quick power shift cycle is a limited supply of perishable and harvestable food, before it is necessary to farm, and the limited shelf life of gasoline.
If the event is a worldwide collapse, rather than a national collapse, you can expect the same power shifts to happen, but the revival will take much longer than 30 years. It will probably take 100 years or more, which means that advanced aspects of civilization, like power plants, industrial equipment, and electronics, will not be learned by a few generations, because the educational materials will be lost and the surviving professionals won't have the ability to teach apprentices. Even if we did have the educational materials and professionals to teach, everyone will be hiding in their own survivor colonies, communications will be disrupted and the loss of a mutual sense of security will prevent the networking and organization, which is necessary for a civilization to exist.