Trump was big on the national stage long before he was president. Why would he go away after the election is over? He’ll still have tens of millions of (probably angry) followers, deep pockets and a huge megaphone.
There has already been some talk of Trump starting his own television network to rival Fox News, and/or his own social media platform — the latter made more plausible by the heavy censorious hands of those running Twitter and Facebook — and I suspect that Trump would regard a 2020 loss as a setback, not a defeat. Grover Cleveland came back to win a second term after losing the White House, Trump might reason. Why not me? He’ll probably hold campaign-style rallies around the country starting right after the election.
Democrats want to get rid of Trump because they think it will get rid of Trump voters. But they’ll still be around no matter what happens in the election, and they have legitimate grievances — lots of them — with the way the country has been run. Likewise, of course, a Trump win won’t get rid of the Angry Left. I’d say that it will just make them angrier, but I think they’ve already gone to 11.
I think that the older political coalitions — like the New Deal coalition, which combined Southern Agrarians with northern near-commies (and sometimes actual commies) — tended to mute ideological hatred. Intraparty battles could be fierce, but it was still all in the family, and party leaders could work a compromise. Now the parties are much more ideological. The funny thing is that many years ago — long before I was born — the American Political Science Association called for more ideologically pure parties. And now that’s what we’ve got. As usual, when you listen to political scientists, the result is disaster.