Tester’s Senate Fate Could Make or Break a Harris Presidency

On the day Barack Obama took over the White House on Jan. 20, 2009, six of his cabinet nominees were immediately confirmed by the Senate. He signed his first piece of legislation — a major bill guaranteeing equal pay for women — into law just nine days later.

Should Vice President Kamala Harris win the presidency, she could not count on such cooperation from the chamber where she, like Mr. Obama, once served. Mr. Obama benefited from a big Democratic majority in the Senate. But Democrats are in control now by only the slimmest of margins, and their chances of keeping that majority most likely hangs on the fate of Senator Jon Tester of Montana, who is currently trailing in his re-election race in his solidly red state.

If he should lose and Democrats fail to score any upsets in a handful of races they are not favored to win, Republicans would take over the Senate, putting Ms. Harris at loggerheads from the start with a newly empowered G.O.P. bent on stymying her at every turn.

“It is night and day,” Senator Richard Blumenthal, Democrat of Connecticut, said about the difference between his party hanging on and Republicans winning the Senate. “It’s loss of control, putting the agenda very much in peril.”

At the moment, most analysts lean toward Republicans capturing the Senate, given a political map that was stacked against Democrats from the start and has only gotten tougher for them. The G.O.P. is all but certain to win the West Virginia seat being vacated by Senator Joe Manchin III. And Mr. Tester is lagging in a state expected to vote overwhelmingly for former President Donald J. Trump.

The swing of those two seats alone would be enough to knock Democrats out of their 51-to-49 majority and fundamentally alter the governing