I'll break down my answer into county sizes. This answer is my best swing at what would happen if the government manages to hold on at least a bit. Maybe they use messenger pigeons. They have only been out of use for less then 100 years. I assume that the allies of the US would work to deter the attacker, so I didn't include a US response to the threat.
County A- Urban City
The first night of the power outage will likely resemble a mixture of the first Siege of Sarajevo, a 1400 day siege in the '90s, and the riots that we see on TV in cities today.
At first, if any order is to be held quickly it will likely be done with fear. Police and Local Militias (Coast Guard) even potentially Military support from cities near Military bases, will try their best to contain potential threats by putting major cities under siege. In the actual streets anarchy will likely rule until the government is able to squeeze in on the rioters and people left in the city. A few people might survive by hunkering down, boarding up windows and doors, with food and weapons for protection, but if it resembles the Siege of Sarajevo and it is years before the military support can step in, everyone will be forced into the streets.
One alteration from this would be time of year. Even in Chicago, IL, where people are shot almost everyday, will experience 0 murders in the dead of winter. If the elements force people indoors, control might be attainable for cities that experience heavy snowfall and sub freezing temperatures. Even if the outages start in the summer, the Gov. would just need to hold out until the Winter to move in on cities like Milwaukee, Chicago, Denver, New York etc.
I conclude this section by realizing war crimes will most likely be committed,for example chemical warfare would allow the military to move in and take action faster.
County B- Large suburbs
Large Suburbs are likely where the Gov. presence would retreat to/ congregate. Check points and barriers would be set up, and local eligible people would be recruited to work on security, or assigned jobs by the Gov to aid the effort of control. The people who have not fled would likely take the positions and leadership, especially if they were to offer food and protection.
County C- Small suburbs/ Large Rural
Small suburbs would fall in between the small pop rural and the suburbs. Most likely these size cities would be extorted for the growing and animal husbandry potential, being closer to the military power that needs the food then the rural peoples. Cities with larger police forces and previously active local governments might find they can self sustain, trading with the army food for weapons/ gasoline/ cigarettes.
County D- Small Pop Rural
The lack of people would allow for a higher potential of survival, aided by the people who live in low pop rural areas are often those who provide food for the rest of society. Small units would form of farmers and locals, who would start their own societies for better protection and resource management.
They might be ignored by the tanks and jeeps that the military roles out to the cities at first, but will be aware that soon attention will turn to face them for their resources of food. They might be able to operate independently because they will have the most valuable trade items in the country, and likely have enough fire power to keep away looter and bandit gangs. It is also common culture in rural low pop towns to have a sense of being self reliant as access to resources people have in the cities or even in suburbs would not normally be available to them. For instance, they are more likely to know how to fix up an old tractor, and have spare gasoline barrels in a barn. Even luxury item farms, like tobacco farms, would still be able to trade their product to get ahead in the world. I could see slavery returning, although it would be enslavement of looters from the city/ suburbs not race based.