The apparent good news for Democrats is that they are likely to win the majority of Senate races next year. If you sum up the percentage forecasts across all 33 races, it comes out to 18 or 19 wins for Democrats, versus 14 or 15 for Republicans.

The problem, of course, is that that Democrats were already in possession of 23 of the 33 seats. If they retained just 18 or 19, they would lose a net of 4 or 5 seats to Republicans — enough to cost them control of the chamber.

Thus, I concur with the conventional wisdom that Republicans are favorites to win control of the Senate next year.

What’s more difficult to do with an informal technique like this one is to estimate how heavily Republicans are favored. There’s a big difference between being a 90 percent favorite and a 55 percent favorite.

My impression is that people tend to under-appreciate the degree to which the outcomes in different Senate races are correlated. If the political environment is strong overall for the Democrats next November, they could win all or almost all of the competitive races. Conversely, they could lose all or almost all of them if the political winds are blowing in a Republican direction.