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.