The battle to pick the Republican nominee for U.S. Senate in Alabama is headed to a March 31 runoff.

Jeff Sessions, President Donald Trump’s former attorney general who served as senator until 2017, will face former Auburn University football coach Tommy Tuberville.

Sessions should have been a shoo-in for his old seat. He represented the state in the Senate for 20 years before joining the Trump administration.

He was the first senator to endorse Trump, lending him establishment credibility and opening the doors to the political world during the 2016 election. Trump, in turn, rewarded him with a job as his attorney general.

But Trump soured on Sessions after he refused to act as Trump’s personal protector and recused himself from the investigation into Russian interference in the 2016 election. He has called Sessions an “embarrassment to the great state of Alabama” and said the Justice Department was a “disaster” when he ran it.

Vasha Hunt/Associated Press Jeff Sessions was once one of President Donald Trump's closest allies — and became his attorney general — but they had a falling-out when Sessions recused himself from the Russia investigation.

The Senate race was very much about who could tie themselves closest to Trump. Both Tuberville and Rep. Bradley Byrne (R) went after Sessions for not being loyal enough to the president.

But Byrne and Tuberville have also criticized the president in the past. In September, Tuberville said he was “pissed off” at Trump over veterans’ health care issues, and Byrne said Trump was “not fit to be president″ after his “Access Hollywood” comments about sexual assault came out in 2016.

Sessions nevertheless emphasized his connection to Trump, calling himself his “No. 1 supporter.” The president didn’t weigh in or endorse anyone in the primary.

The winner of the GOP primary will face off against Sen. Doug Jones (D-Ala.). Jones won the seat when Sessions resigned to join the Trump administration. Alabama is deep red, and it seemed like a safe guess that Republicans would be able to hang on to the position. But the GOP nominee, Roy Moore, faced accusations of sexual assault involving teenage girls, and Jones was able to pull off a win. Republicans see Jones as a top target to defeat in 2020.