No Stupid Questions

Not even close to being done. Right now the biggest changes are a reduction in non law enforcement/immigration government staff and contracts being paid out. The biggest thing coming down the pipeline is Trump clearly wants to free himself of the courts and congress. But it's far too early to say he's won that. And even that wouldn't be the end of things. In the US the states have a lot of autonomy. They are actually the ones responsible for holding elections. So let's look at a worst case scenario, where he tries to say we shouldn't have elections.

The first thing that's going to happen is all the blue states are going to tell him to fuck off and hold them anyways. The second thing that's going to happen is some red states will also do so, although they'll likely be less coarse with the language. Then a few more red states will be pressured into having elections by massive protests of people angry they can't vote anymore. Then while Trump is having a fit because there's no real way for him to stop this process, we get to learn about a fun feature of the US Congress. There is no law requiring it to meet in D.C. Trump would likely try to claim whatever is left over is the real congress, but without having been elected the Constitution is clear that those states forfeit representation until they hold an election.

So we'd be left with a House that is majority anti-Trump, after all, he tried to make them irrelevant at best. In the Senate we'd likely be looking at something of an even split in 2026. There's probably 5-7ish red states that would hold elections anyways and combined with the blue states and senate democrats leaving DC they would be able to convene elsewhere with a majority to declare rules of the Senate without Trump's interference. The new Congress would likely swiftly vote to impeach Trump. The remnants of the old one would protest this but they don't have any legal power. Only the backing of Trump and propaganda power.

This leaves Vance with a choice. This would be by design because our democratic party leaders only appear to be stupid when convenient. Vance can throw his weight behind Trump and get impeached himself or he can order Trump removed from the White House thus acknowledging the primacy of Congress. If he chooses the first option then Congress simply repeats the process and the presidency goes to the next person in line, the speaker of the house. Yes, Congress can effectively vote one of it's own members into the White House at any time. This president then declares an emergency and orders the military to secure DC. The military loves process, and loves the Constitution. It is highly likely this order would be followed.

However all would not be well, it's not a fairy tale. It would likely be the start of an American Insurgency that would take decades to root out. It would certainly be the end of the US as the hegemonic world power. Our Aircraft Carriers would rust in port and our projection of soft and hard power over the world would wither. But we would still be here, just much diminished and never the same in our lifetimes. This is certainly scary but if we all do our part this is as close as we would come to losing our democracy. Far more insidious is the threat of slowly revoking the right to vote. They'd start by raising the age, then by requiring you to not have any debt of specific kinds, then by making harsh punishments for illegally voting, and other such things until voting is effectively restricted to land owners. Certain factions would like to get it to white christian male landowners but that's probably a decade or more down that line if at all.

Notes -

Why wouldn't he just send in the military?

2028 isn't enough time to purge and train enough people to make the military loyal to him. He would be mid project on that at best and the states could effectively counter him into a stand still with their national guard. This would make many people stay home, but the determined voters are likely to be anti-trump because that's the change incentive. Loyalists will feel like the elections don't matter.

What's stopping SCOTUS from declaring the elections invalid?

The states. SCOTUS is only relevant as long as they have reputation of being an impartial arbiter of Constitutional Law. That opinion is already in the trash heap. They could not make such a decision today, or after 4 more years unless they spend the next 4 years setting themselves as at least a mild opposition in a long game. But they haven't shown that kind of patience.

What happens in Trump surrounds himself with thousands of armed loyalists in DC?

We select a new capital and wish him the best of luck dealing with DC. There is no law requiring DC be the Capital. The Constitution doesn't even require the states to give up a district, it only provides the legal possibility. There's no need to engage in that kind of a conflict. Such a group would be arrested bit by bit by Maryland, Virginia, and Federal authorities until it could be resolved swiftly.