Jeff Sessions. Chip Somodevilla/Getty Images White House chief strategist Steve Bannon helped convince Attorney General Jeff Sessions to endorse Donald Trump in the 2016 election despite Sessions' worries that the endorsement could be the end of his political career, a new book revealed.

As Joshua Green wrote in "Devil's Bargain," Sessions, then a senator from Alabama, was unsure if Trump could secure the Republican nomination, and knew that being the first senator to endorse Trump could further curtail his political future if Trump, the Republican frontrunner at the time, lost.

The day before Sessions endorsed Trump at a Madison, Alabama rally in February 2016, then-Breitbart News chairman Bannon told Sessions that it was "do or die" time and that "this is the moment" to endorse.

"Trump is a great advocate for our ideas," Sessions told Bannon. "But can he win?"

"100%," Bannon said. "If he can stick to your message and personify this stuff, there's not a doubt in my mind."

Sessions then noted that the GOP already denied him the chairmanship of the Budget Committee, and that "if I do this endorsement and it doesn't work, it's the end of my career in the Republican Party."

"It's do or die," Bannon replied. "This is it. This is the moment."

That moment was just days before what are known as the "SEC" primaries — a series of primary contests concentrated throughout the South. Bannon told Sessions that his endorsement could push Trump over the hump in many of those contests and essentially seal up the Republican nomination.

"Okay, I'm all-in," Sessions said. "But if he doesn't win, it's over for me."

One of Trump's most prominent backers since that moment, Sessions now finds himself the target of a week's worth of broadsides from the president.

Throughout the past week, Trump has expressed displeasure with Sessions' recusal from all investigations involving the Trump campaign, which allowed for Deputy Attorney General Rod Rosenstein to appoint special counsel Robert Mueller to oversee an investigation related to whether members of the Trump campaign colluded with Russian officials in the 2016 election. Rosenstein appointed Mueller after Trump abruptly fired James Comey as FBI director in May. Comey was overseeing the FBI's investigation into Russian election interference.

On Wednesday, Trump fired off a pair of tweets asking why Sessions has not "replaced" acting FBI Director Andrew McCabe. The president started his day on Tuesday by blasting Sessions on Twitter for having taken "a VERY weak position on Hillary Clinton crimes (where are E-mails & DNC server) & Intel leakers!" The president also asked "where is the investigation A.G." in a tweet in which he discussed "Ukrainian efforts to sabotage Trump campaign."

In an interview with The Wall Street Journal later that day, Trump said he was "very disappointed in Jeff Sessions," a statement he would repeat in a Rose Garden press conference alongside Lebanese Prime Minister Saad Hariri just hours later.

Pointing to Sessions' February 2016 endorsement, Trump told the Journal it was "not like a great loyal thing."

"I had 40,000 people," Trump said. "He was the senator from Alabama. I won the state by a lot, massive numbers. A lot of the states I won by massive numbers. But he was a senator. He looks at 40,000 people and he probably says, what do I have to lose, and he endorsed me. So it’s not like a great, loyal thing about the endorsement."

Trump has not, however, said whether he plans to oust Sessions.