Here's one more reason for Republicans to be very nervous about the Senate in 2018

AP

Doug Jones

• Doug Jones' upset victory in Alabama on Tuesday night provided more reason for Republicans to worry about the 2018 midterm elections.
• Jones turned out a remarkable number of voters in an "off" year to squeeze out a victory over Moore.

Here's the most remarkable statistic about Tuesday's election in Alabama: Doug Jones turned out nearly as many voters as Hillary Clinton did in the state last year, while Roy Moore got only about half as many as Donald Trump had gotten.

The circumstances in this election were extreme, and the results were an extreme manifestation of a trend that we've seen in special elections across the country: Democrats have been outperforming in large part because Democratic voters are turning out much more strongly than Republican ones.

This should make Republicans very nervous about next year.

There is no sign that Trump voters will be there when they are needed to help Republicans retain Congress - even if Trump tells them to show up.

Yes, most Republican candidates won't be nearly as damaged as Moore. But most elections won't be in places as fundamentally favorable to Republicans as Alabama, either.

Will Republicans vote in Senate elections next year?

Thirteen months ago, most political observers would have told you the 2018 Senate map was so unfavorable to Democrats, they would be almost sure to lose seats. Democrats will defend Senate seats in 10 states won by Trump, including five where he won by more than 10 points.

The Democratic incumbents typically described as most endangered are Claire McCaskill in Missouri and Joe Donnelly in Indiana, states that Trump won by 19 points each. (He won Alabama by 28 points.)

Democrats will also have to defend unfavorable terrain in North Dakota, Montana, and West Virginia, but in these states Democratic incumbents are thought to be more protected because they have built political profiles that closely resonate with their red-state voters.

I would note, while most states will hold elections for governor next year, it so happens that none of these five states will do so. Which is to say, if voters turn out in these states, it will be foremost in order to vote in the Senate race.

If Democrats could gain such a turnout advantage that they managed to win a Senate race in Alabama, is it hard to see them doing so in less-red states like Missouri and Indiana, where their candidates are incumbents who have already demonstrated an ability to win statewide?

Democrats are one seat closer to a majority

Since Jones' term runs through 2020, Democrats need to pick up only two seats to gain a Senate majority. And they have two strong opportunities - an election in Nevada, which Clinton carried, and an open-seat race in Arizona, where Republicans are likely to have a nasty primary and may end up with a problematic nominee.

If Democrats pick those two seats up and defend all their existing seats - and if they win a special election to retain Sen. Al Franken's seat in Minnesota - if, if, if - they will end up with 51 Senate seats.

That's a lot of things that have to break the right way. But this is not like flipping a coin seven times and hoping it always comes up heads. Election outcomes in a given year are heavily correlated - if Republicans have a bad night in West Virginia, they're likely to have a bad night in Arizona and Missouri at the same time.

Democrats also have outside chances to pick up Senate seats in Tennessee and Texas, though Republicans are heavy favorites both of these races.

And though it's not pleasant to discuss the possibility, there may be a special election for a senate seat vacated by Arizona's John McCain in 2018. When a state holds two Senate elections at the same time, they almost always go both to the same party. The last time a state rendered a split verdict in a double Senate election was in 1966, in South Carolina - and in that year, voters chose Fritz Hollings, a Democrat, and Strom Thurmond, who had been a Democrat in his prior races.

All of which is to say, over the last year, the path to a Democratic senate majority in 2018 has gone from outlandish to plausible. The PredictIt prediction market - where participants wager real money on election outcomes - gave Democrats a 41% chance of retaking the Senate next year as of 9:15 a.m. on Wednesday.

Donald Trump is more unpopular than any president at this point of his term in the history of modern polling. Republicans are rushing to enact one of the most unpopular pieces of major legislation in memory. We're seeing evidence of the toll those facts are likely to take on the party in elections up and down the ballot.