Over at Tradesports?

According to the betting at Tradesports, the Republicans have a 66-69% chance of retaining the Senate, which is down from a few days ago.

Yet the Democrats are ahead in all of Tradesports’ closest Senate races...

Probably not. Of course, as the Lindgren notes, and is discussed extensively in comments, the individual races are individual, whereas the Senate overall is a joint probability of them. And there's little way to figure out how independent they are of each other.

Well, looks like the Democrats picked up enough to carry the Senate too. The Senate odds are now 7-9% that the Republicans keep a majority. TradeSports blew that call.

Suppose there were two races and you need 1 of 2 to win the Senate. It is reasonable to assume that the races are independent since there is no logical means of them anti-correlating which would be the only way that a win by one would make less likely a win by the other. Suppose Dem X is trading at 55 cents and Dem Y is trading at 55 cents and Dem Senate is trading at 33 cents. Suppose that there is no spread so that Rep X is 45, Rep Y is 45 and Rep Senate is 67.

If you buy Rep X, Rep Y and Dem Senate, you are guaranteed to get at least $1. The trouble is that costs $1.23. If you believe that Rep X and Rep Y are almost completely correlated, you can buy 1 share of Rep X, 1 share of Rep Y and 2 shares of Dem Senate for $1.56 and almost always make $2.

If the two seats are completely uncorrelated, then Dem Senate should be trading at 79, not 33. If they are completely correlated, it should be trading at 55, not 33. So when you see this, buy Dem Senate and partially hedge by buying Rep X and Rep Y. Look for a chance to cash out before the election is resolved. Not an arbitrage opportunity, but a pretty good bet.