At this time last year, the Democratic path to Senate control seemed impossible: Hold all of the Democratic seats, flip Arizona and Nevada, then hope for a miracle.
The Democrats got the political version of a miracle on Tuesday. Doug Jones’s victory in Alabama means Democrats have accomplished the most difficult item on their checklist in pursuit of the Senate. A Democratic path is now obvious, and the race for control is basically a tossup, perhaps with a Republican advantage.
It is hard to overstate how surprising this would have seemed a year ago. Democrats needed three states to flip control of the Senate, but they entered the cycle defending 25 seats (two of them independents) to the G.O.P.’s eight. Of those Democratic seats, a staggering 10 of them were in states that chose Donald J. Trump for president, including five that he carried by at least 18 percentage points.
Only one Republican, Dean Heller, represented a state (Nevada) won by Hillary Clinton. Jeff Flake’s seat in Arizona was also plausibly competitive after Mr. Trump’s tepid 3.5-point win in the state, but it was hard to find the third Democratic seat. Perhaps the next best Democratic opportunity was against Ted Cruz in Texas — a long shot at best.
But the Republican position has steadily deteriorated throughout the year. Most obviously, Mr. Trump’s weak approval ratings have decidedly shifted the national political environment. The party’s two most vulnerable seats — those held by Mr. Heller and Mr. Flake — became much more vulnerable. Mr. Flake said he wouldn’t run for re-election, while Mr. Heller came out of the health care debate badly damaged and facing a primary challenge.
The retirement of Bob Corker in Tennessee and the entry of a former Democratic governor, Phil Bredesen, gave Democrats a new, credible option for that crucial third seat. Texas remains a long shot, but the likely Democratic nominee, Beto O’Rourke, has run a vigorous and well-funded campaign. Republicans did get some good news in Al Franken’s retirement, but Democrats would clearly be favored to hold a Minnesota seat in this political environment.
At the same time, it became clear that the Republican opportunity to flip Democratic-held red states wasn’t as good as it looked.
It increasingly seems that many red-state Democrats are favorites to win re-election, despite the steady national trend toward a tighter relationship between presidential and Senate vote choice. Polls show that red-state Democrats remain popular, and most hold a lead over many of their strongest potential challengers. A tough political environment has so far discouraged many of the G.O.P.’s strongest potential candidates, and those who do run might face tough primary challenges from Steve Bannon-backed insurgents.
Historically, the strength of the red-state Democrats isn’t a surprise. Over the last few decades, incumbent senators representing the party out of power have done an exceptional job of defending seats in midterm elections, even in the most hostile terrain. In 2006, for instance, Democratic senators in Nebraska and South Dakota won re-election by huge margins.
Even if you looked only at the most recent few rounds of House or Senate results, you would still conclude that every Democrat running for re-election in 2018 was an even-money favorite or better in this political environment.
My sense is that Democrats would be favorites in Nevada and Arizona if 2018 turned out to be a so-called wave election like the 2006 or 2010 midterms. This year’s special elections and national political indicators suggest that development is likelier than not.
So the judgment about G.O.P. chances really comes down to whether you believe Republicans can break the pattern of re-election successes by the party out of power, most likely in Indiana or Missouri, versus the Democrats’ chances of winning in Tennessee or Texas.
As an empirical matter, a model-based approach would give the Democrats around a 40 percent chance of winning control of the Senate in a political environment like that in 2006 or 2010, after adjusting for the trend toward the greater relationship between presidential and down-ballot vote choice. (That doesn’t account for the possibility of additional Republican vacancies.)
If Mr. Jones had not won, it would have been a very different story.