Ms. Heitkamp’s announcement is likely to push other moderate Democrats facing re-election in states that Mr. Trump won to follow suit. Many of those senators — including Joe Manchin III of West Virginia and Claire McCaskill of Missouri — voted for Mr. Pompeo last year when he was confirmed to be C.I.A. director, and they are under renewed pressure at home to show they are willing to vote with Mr. Trump’s interests from time to time.

But before an expected full Senate vote, the Foreign Relations Committee — with 11 Republicans and 10 Democrats — is scheduled to vote on Mr. Pompeo’s nomination Monday evening. Senator Rand Paul, a Kentucky Republican, has said he will vote against Mr. Pompeo, and all 10 Democrats appear likely to do the same. The committee’s chairman, Senator Bob Corker, a Republican from Tennessee, has vowed to send Mr. Pompeo’s nomination to the full Senate anyway…

The Senate Foreign Relations Committee has never given a nominee for secretary of state anything but a favorable vote since the committee began considering nominees in the late 19th century, the Senate historian said on Thursday.