I’ll also stick in a plug here for PredictIt, which is pretty liquid on all of these things. The current market for Trump is 42/43; the
market for any Republican is 45/46; for any Democrat it’s 56/57; and if you like free money the Libertarian and Green parties are both still 2 bid.
(My fave trade in this particular market is shorting Andrew Yang, who’s inexplicably constantly bid around 10 or 12 to win the Democratic nomination despite never polling above 3% in any poll ever. Shorting Tulsi Gabbard at 5 was good money too, but she collapsed to 1 bid / 2 offer when she missed out on the upcoming debates, so there’s no more juice left in that trade.)