There are a few ways this can happen.
1) Trump voluntarily steps down and leaves Pence in charge. For example, if Pence was to pardon Trump and all of Trump's family and a bunch of Trump's friends and colleagues. We would be paying for a peaceful transition with blatant, obvious corruption.
Trump
would do this, but not soon. This seems like the sort of thing he'd do Jan 19th around 11PM.
2) The stress of the election gets to Trump and he has a heart attack, stroke, syphillis flare up, or other medical issue that takes him out. Or, possibly,
fakes this to secretly #1 above and make it look like this wasn't his plan. Based on the tweets and speeches we've seen, this is the most likely of the options but still not very likely.
3) Trump's actions get so illegal and so violent that the 25th is applied. Trump's cabinet is stuffed with yes-men who are apparently telling him not to concede, but, Trump could literally start bombing Iran, ordering federal troops to seize Philly, or tell his supporters to attack voting centers without subterfuge. The line Trump would have to cross to force this is faint and far away, but it does exist. We don't want to see it.
4) Or, as @
Doctor Amadeus Amadeus, Amadeus Amadeus Amadeus, Amadeus Amadeus Amadeus, oh oh oh Amadeus said, he gets impeached for basically the same reason. McConnell would be more likely to do this than Trump's cabinet, but barring massive violence or blatant theft he'd still be highly reluctant to do so. It would be admission of massive failure, even though he personally is safe for six years, his friends aren't.
5) And finally, and despite how often this comes up in these forums I find this highly unlikely, Trump could flee to Russia to avoid prosecution. This would force a delayed reaction to the above, since Trump's enablers would say "Well I'm sure he'll be back tomorrow...I mean in a few days...next week..he does plan to return eventually, Kif."
Trump is a narcissist, and a cult leader. He won't cede power willingly. So, wait for him to keel over, or wait for him to do something massively dangerous, before this happens.
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This is, what, two or three groups of armed Trump supporters trying to commit crimes but being ratted out?