The betting markets at PredictIt.org currently suggest that Republicans might pick up a net 3 seats in the U.S. Senate.
The markets show Republicans with a better than 50% chance of picking-up seats in Missouri (McCaskill), Florida (Nelson), North Dakota (Heitkamp), and Indiana (Donnelly).
Only in Nevada (Heller) are Democrats given a better than 50-50 shot of flipping a seat.
The Republicans are also defending open seats in Arizona and Tennessee. As of 10:00 a.m. Eastern on September 4, the GOP was favored to hang on to both (although the Arizona race is just barely better than even for Republicans).
Obviously, the betting markets are not perfect. But, especially as we head into the fall campaign season, they are a pretty good reflection of the conventional wisdom for each race.