If you look at the individual Senate races at Tradesports, then compile the results, you wind up with a prediction that the GOP will lose the Senate.
That would be assuming all the Senate races were independent. It would be possible for the odds of every individual race to be 60% against, and yet the odds of winning more than half of them to be 60% in favor. As a scenario, divide the races into five groups A-E and roll 1D8.
If the roll is 1, win races ABC.
If the roll is 2, win races BCD.
If the roll is 3, win races CDE.
If the roll is 4, win races ADE.
If the roll is 5, win races ABE.
If the roll is 6-8, lose them all.
Each group of races has only a 37.5% chance of victory. The overall chance of winning, however, is 62.5%.