WASHINGTON (Reuters) - In a blow to former attorney general Jeff Sessions’ attempt to reclaim his seat in the U.S. Senate, President Donald Trump on Tuesday endorsed his challenger for the Republican nomination in Alabama, Tommy Tuberville.

“He will be a great Senator for the people of Alabama. Coach Tommy Tuberville, a winner, has my Complete and Total Endorsement,” Trump wrote in a post on Twitter.

As a senator from 1997 to 2017, Sessions was an early congressional backer of Trump’s bid for the 2016 Republican presidential nomination.

Trump, a businessman turned reality television personality, was considered a long shot in the race. After his surprise victory, he named Sessions the attorney general of the United States, the nation’s top law enforcement official.

Although the two were closely aligned on Republican priorities for his presidency, Trump soured on his onetime ally after Sessions recused himself from overseeing the FBI’s probe of Russian interference into the 2016 election.

Sessions’ recusal led to the appointment of Special Counsel Robert Mueller, who headed a sprawling, nearly two-year long investigation into the subject. The investigation ultimately detailed how Trump tried to derail the inquiry but stopped short of concluding the president had committed a crime.

Trump has called his selection of Sessions as attorney general “the biggest mistake” of his presidency.

Sessions will face Tuberville, a former football coach at powerhouse Auburn University, on March 31 in a runoff to become the Republican nominee for the Senate seat and face incumbent Democrat Doug Jones in November.

Jones was the first Democrat in a quarter of a century to win a Senate seat in Alabama.

The Republican party holds a narrow majority in the U.S. Senate, with 53 out of 100 seats.