President Trump declared his support Thursday for Sen. Cindy Hyde-Smith (R-Miss.), wading into a closely watched race that has divided Republicans.

In a pair of late-afternoon tweets, Trump wrote that Hyde-Smith has helped him “put America First!” and has his “complete and total Endorsement.”

Hyde-Smith was appointed earlier this year to succeed Republican Thad Cochran, a longtime senator who stepped down amid health problems. She is running in a special election that will be held on Nov. 6, the same day as the regularly scheduled midterms.

Republicans are defending a slim 51-49 Senate majority — making every race a potentially decisive contest in the battle for control of the upper chamber of Congress.

Also in the Mississippi race is Chris McDaniel, an insurgent conservative who has been hostile to Senate GOP leaders and nearly defeated Cochran in 2014. McDaniel was encouraged to run by Stephen K. Bannon, Trump’s former chief strategist. He was a supporter of Sen. Ted Cruz (R-Tex.) in the 2016 presidential primary.

Candidates of all parties will compete on the same special election ballot in Mississippi. If none of them receive a majority of the vote, the top two finishers will head to a runoff.

Some Republicans have worried that if McDaniel, a polarizing figure, advances to a runoff against a Democratic candidate, the seat would be at risk of flipping from red to blue.

Trump’s endorsement has proven to be a valuable asset to Republican candidates in primaries this year. While Trump is a deeply unpopular figure in many areas of the country, in ruby-red Mississippi, Republican strategists believe he can be a powerful motivator.