Hey MT. I'd be really interested in yr opinion on the likelihood of Trump winning again in 2020. I don't know the US system well enough to understand if any threat is as likely to come from the GOP as much as from the Dems. Anyway you can get 66/1 on Eminem so I might go for that as an outsider
I would say he is a slight favourite against the field. His re-election fund has already raised over $100m, which gives him a big advantage, and voters seriously favour incumbents anyway.
His own party is a threat - not so much that they made topple him (though they could), but that they could weaken him so much that he loses the General Election as a result. There have been three proper attempts to dislodge the sitting President from within his party in recent times. Ronald Reagan challenged Ford in 1976, Ted Kennedy stood against Carter in 1980 and Pat Buchanan took on Bush 41 in 1992. Buchanan didn't get close, but Reagan and Kennedy took their challenges all the way to the Convention, and Reagan only lost by a whisker. Although they held off their internal challengers, all three Presidents went on to be beaten in the General Election.
It's not impossible to dislodge a President from within. Johnson withdrew from the race in 1968 cos he could see that it wasn't going his way and Reagan probably would have beaten Ford if it wasn't for a tactical error at the Convention. After the result, he gave a masterful speech that resulted in many delegates concluding they had voted for the wrong candidate.
Ted Cruz, Marco Rubio, Mitt Romney, Rand Paul, Mike Pence and Paul Ryan have all said they won't challenge Trump. They would probably all lose to him anyway. Jeff Flake, Ben Sasse and Bob Corker want to have a go, but it's hard to see them doing well. One Republican who I think could beat him is Nikki Haley, but she has also said she's not going for it. So that really only leaves one person who has a chance of being successful and wants to run - John Kasich. He put up a good showing last time and is still hugely popular and respected in his State, the Party and the nation. If Trump struggles against Kasich, it could signal the end of the road even if he wins.
All this is assuming he runs. House Committees will be Democrat-dominated from now on, and they could start investigations into all sorts, including Trump's personal finances, so his position might be much weaker by 2020. There is also speculation that he may decide he has achieved most of what he set out to do, or that it's just not worth continuing to put himself and the family through it all for years to come.