Predictit.org is already permitting wagers on the 2020 elections. Here are a few punts, and the probability of who will win based on the bets.
Democrats win presidency: 63%
Republicans win presidency: 37%
Republicans control Senate: 55%
Democrats control Senate: 45%
Democrats control House: 69%
Republicans control House: 31%
Democratic Nominee for President,
Kamala Harris 23%
Joe Biden 16%
Bernie Sanders 15%
Beto O'Rourke 13%
Elizabeth Warren 10%
Cory Booker 8%
Amy Klobuchar 6%
https://www.predictit.org/markets/1/US-Elections