Trump is defeated and the Senate flips Democratic. It is assumed the House remains in Democratic hands.
The Supreme Court session begins 5 October, 2020, presumably with the new Trump appointee sworn in by 1 November. The election follows just a few days later. The new Senators are sworn in 3 January, 2021. The new President is sworn in 20 January, 2021
Trump has been casting out feelers about challenging the results of the election if it appears he is going to lose, or if the results indicate he has lost on election day. There are a variety of ways he might try to pull this off, all of which he has mentioned himself.
He may decide to postpone the election all together. He may refuse to accept the results, citing ballot-box stuffing, foreign intervention, Democrat malfeasance, illegal alien voting, the denial of Conservative precinct results, MSM fake news or some other crimes by his enemies. He will have almost two months between election day and the Senate inaugural ceremonies to try to steal it and a month after that to consolidate his victory before the next inauguration. And he WILL try, you and I both know he will not accept a loss unless he is convinced he has absolutely no choice. And he has already psyched himself, and a large proportion of his followers, that he can subvert the election.
Between early November (or whenever the election is certified), and late January he will still be President, he will have control of the Senate and the Supreme Court, and he will have his Justice Department under AG Barr on his side, not to mention Homeland Security and ICE. Other resources will be the State Houses and Governors controlled by the GOP (or who will fear losing their jobs as a result of the election), and many of the Federal and State Courts the Republicans have been systematically stacking for years.
Not all of these can be counted to back him, but all can be counted on to look the other way…
He will also have on call his irregular militias, skinheads, paramilitaries, the NRA, motorcycle gangs, bible-bangers, assorted racists, rogue cops and right wing ex-GIs and the fascist media and talk radio. True, not all these groups are well organized or coordinated, but even if just a few come out in force their actual effect will be magnified and amplified by the press coverage. Its tiki-torch time in Amerika. He may not be able to pull it off, but there is no doubt in my mind he will think he can, and if he convinces himself he can get away with it, he will try to do so. And if you believe he will get little support from the Republican Party, just remember what happened with Garland, Mueller, the Impeachment, the Kavenaugh hearings and now the Bader-Ginsburg replacement. RBG hadn’t even been buried and McConnell had already worked out the details. Any doubt I may have had vanished when I saw how quickly, completely and obediently the Right came together on the SCOTUS vote.
“Democrats fall in love, but Republicans fall in line.” They will fall in line. You can count on it. The only thing that can stop it is a Biden landslide in November, an overwhelming victory that will make Trump lose his nerve. The one thing we have going for us is that, like all bullies, he is a coward.