Live Updates on Elections in Maine, Alabama and Texas

Since that tweet, Trump-endorsed candidates have been on the losing side of four Republican primaries, falling short in Colorado, Kentucky, North Carolina and Virginia. Now Tuesday’s contests bring three more high-profile primary runoff contests that pit Mr. Trump against pillars of the local Republican Party.

The highest profile race is in Alabama, where Jeff Sessions, the former attorney general, is a long-shot to win back the Senate seat he held for 20 years. Mr. Trump, still embittered by Mr. Sessions’s decision to recuse himself from the Justice Department’s investigation into Russian election interference, endorsed Tommy Tuberville, the former Auburn University football coach.

A Sessions victory would represent a major black eye for Mr. Trump. There is no Republican primary candidate for whom he has campaigned harder this year than Mr. Tuberville. Along with the many tweets denouncing Mr. Sessions, Mr. Trump planned and then canceled an outdoor campaign rally with the former coach and did a Monday night phone call touting Mr. Tuberville.

In Texas, Mr. Trump endorsed in a pair of House contests to be decided Tuesday.

In the Panhandle, he’s backing Ronny Jackson, his former White House physician, against Josh Winegarner, a local lobbyist who is endorsed by Representative Mac Thornberry, who is retiring after 13 terms. The primary winner is certain to come to Congress — the district is among the most Republican in the country.

And in southwest Texas, the president backed Tony Gonzales, a former Navy cryptologist, against Raul Reyes, a former Air Force officer who has an endorsement from Senator Ted Cruz. Representative Will Hurd, who narrowly won re-election in 2018, chose to retire rather than compete in another race against Gina Ortiz Jones, a Democrat. Mr. Hurd also endorsed Mr. Gonzales.

Of course if there is any place where Mr. Trump might be nervous about an endorsement, it would be Alabama. It was there, in 2017, that he found himself on the losing side of the same Senate race twice: First, when Roy Moore defeated the appointed Senator Luther Strange in the primary to replace Mr. Sessions, and then again when Mr. Moore lost the general election to Doug Jones, a Democrat, who awaits the winner of the Tuberville-Sessions race.