President Trump on Tuesday night endorsed Tommy Tuberville, the former college football coach who won the most votes in last week’s Republican primary for the Senate seat from Alabama and is locked in a runoff against Jeff Sessions, the former attorney general.

The endorsement came after Mr. Trump agreed to sit on the sidelines during the crowded primary, which had become a show of which candidate displayed more fealty to the president. Mr. Tuberville received the most votes, and polls show him with a comfortable lead heading into the runoff against Mr. Sessions, which is scheduled for March 31.