Former Attorney General Jeff Sessions is fighting for his political life on Tuesday as he faces a tough run-off for his former Alabama Senate seat, in which President Trump is backing his opponent -- demonstrating how Sessions has gone from one of Trump's earliest and strongest backers to an outcast in Trumpworld.

Sessions will face off against first-time politician and former Auburn University football coach Tommy Tuberville – who has Trump’s backing. As a result of Trump's backing, Tuberville is the favorite to win on Tuesday night.

TRUMP A MAJOR FACTOR IN TUESDAY'S PRIMARIES AS EX-AG SESSIONS FIGHTS FOR POLITICAL LIFE IN ALABAMA 

It’s another demonstration on how dramatically fortunes can change in the Trump White House, and how fickle Trump’s loyalties are to even his keenest of supporters.

Sessions was the first senator to back Trump in 2016, backing him days before Super Tuesday and giving Trump much-needed conservative credentials as they were being challenged by his rivals on issues such as immigration.

Sessions had been a hardliner on immigration, Trump’s signature campaign issue, for years. In a 2015 immigration handbook, the then-Alabama senator called “immigration reform” a “legislative honorific almost exclusively reserved for proposals which benefit everyone but actual American citizens.”

SESSIONS BRUSHES OFF TRUMP'S 'JUVENILE INSULTS'

Getting Sessions’ backing was proof for many grassroots conservatives that Trump’s tough rhetoric on immigration was not just bluster. Sessions was also known for his conservative stance on law and order -- something Trump was running on in 2016, and still tweets about regularly.

When Trump eventually secured the nomination, and then the White House, Sessions was rewarded with the top job of attorney general, where he immediately set to work on those priorities.

He streamlined the hiring of immigration judges at the border, targeted “sanctuary cities,” and announced a "religious liberty task force." On crime, he repealed Obama-era guidance on mandatory minimums, pulled back on civil rights suits against police departments, and was able to brag in 2018 that his DOJ charged the largest number of violent crime defendants in history.

“Violent crime and homicides, which jumped in 2015 and 2016, both dropped in 2017 and will drop again in 2018,” Sessions said in a statement  “There can be no doubt that good law enforcement policies can make our communities safer.”

Liberal critics saw that Sessions was a success by Trump's own standards. A Vox headline declared: "Jeff Sessions turned Trump’s 'tough on crime' dreams into reality."

But, while he was succeeding on his cornerstone issues, he had already angered Trump on the question of the Russia investigation -- which would ultimately prove to be politically fatal for him.

Sessions, who was already under scrutiny from Democrats for two undisclosed meetings with the Russian ambassador during the Trump campaign, announced in early 2017 that he would recuse himself from any investigation into Russian influence in the election. Special Counsel Robert Mueller was appointed shortly afterward.

That move by Sessions infuriated Trump.

"He took the job and then he said, 'I'm going to recuse myself,'" Trump said on “Fox & Friends” in 2018. "I said, 'What kind of man is this?' And by the way, he was on the campaign. The only reason I gave him the job, because I felt loyalty. He was an original supporter."

As the Russia probe rolled on, Trump appeared to be less focused on Sessions’ role enforcing the law and cracking down on illegal immigration, and instead zoned in on Sessions’ refusal to go after Trump’s political opponents.

“Attorney General Jeff Sessions has taken a VERY weak position on Hillary Clinton crimes (where are E-mails & DNC server) & Intel leakers!” he tweeted in 2017.

In August 2018, he tweeted that Sessions should investigate “the corruption on the ‘other side’ including deleted Emails, Comey lies & leaks, Mueller conflicts, McCabe, Strzok, Page, Ohr, FISA abuse, Christopher Steele & his phony and corrupt Dossier, the Clinton Foundation, illegal surveillance of Trump Campaign, Russian collusion by Dems - and so much more.”

“Open up the papers & documents without redaction? Come on Jeff, you can do it, the country is waiting!” he said.

Sessions was fired soon after the 2018 midterm elections, delivering his resignation letter to Trump in which he declared that “we have restored and upheld the rule of law.”

Soon after, he declared his intention to run for the Senate seat he gave up to become attorney general, which had since been taken by Sen. Doug Jones, a Democrat who beat Republican candidate Roy Moore in 2017.

Trump has chosen to support the more moderate Tuberville rather than his former AG, slamming Sessions for months. It's an endorsement that Tuberville has wielded to his advantage to show to Republicans that he is the "MAGA" candidate in the race.

“3 years ago, after Jeff Sessions recused himself, the Fraudulent Mueller Scam began. Alabama, do not trust Jeff Sessions. He let our Country down. That’s why I endorsed Coach Tommy Tuberville (@TTuberville), the true supporter of our #MAGA agenda!” Trump tweeted in May.

Sessions held off from responding, but some of Trump’s biggest backers were infuriated by Trump backing someone they saw as less “MAGA”  than Sessions, and made clear their anger in no uncertain terms.

“The most disloyal actual r****d that has ever set foot in the Oval Office is trying to lose AND take the Senate with him,” tweeted conservative commentator Ann Coulter, who penned “In Trump We Trust” in 2016. "Another Roy Moore fiasco so he can blame someone else for his own mess.”

Recently, Sessions himself has started to push back against Trump’s attacks. On Saturday, after Trump tweeted that “Jeff Sessions is a disaster who has let us all down. We don’t want him back in Washington!” – Sessions returned fire – calling Trump’s tweets “juvenile insults” and emphasizing that “Alabama does not take orders from Washington.”

With the most recent polling suggests Tuberville with a double-digit advantage over Sessions, a top Tuberville supporter acknowledged the importance of the president’s endorsement.

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“Coach Tuberville's work ethic has got him where he's at, but having President Trump come out and endorse him has certainly put the wind underneath his sails because Alabama is Trump country," Alabama House majority leader Nathaniel Ledbetter told Fox News.

The winner in Tuesday's runoff will advance to the general election to take on Jones, who is the most vulnerable Democrat in the Senate in this year’s general election. As the Republicans look to hold onto their 53-47 majority in the Senate, Alabama may be their only real shot of flipping a Democratic-held seat.

Fox News’ Paul Steinhauser contributed to this report.