We probably will not know the final composition of the Senate tomorow. Washington State only counts half of its ballots on election night (the rest are mail-in), so that race probably won’t be called until later in the week — and even then it might be close enough to trigger an automatic recount. There may be recounts in other states, too. But we will get an early indicator of whether Republicans have a shot of taking the Senate.

Jon Manchin, the Democratic governor of West Virginia, is ahead in the polls in that state’s Senate race, a special election to replace the deceased Robert Byrd (meaning that the winner will be seated immediately, in time for the lame duck session of Congress). If Manchin wins, it is vanishingly unlikely that control of the Senate will flip to the GOP, and his apparent strength is a key reason why most analysts are expecting Democrats to hold the Senate even as they lose the House in a historic wave.

Polls in West Virginia close at 7:30 PM Eastern. If John Raese scores an upset against Manchin, there is a good chance that the GOP wave will be big enough to flip the upper chamber. If he doesn’t, there isn’t.