The 73-year-old resigned his long-held Senate seat in Alabama to take the attorney general job in 2017, but after he was made to resign a year later he is now running to regain the seat.
However Mr Trump, who has long held a grudge against Mr Sessions for recusing himself during the Russia investigation led by Robert Mueller, has tweeted support for his primary rival, former college American football coach Tommy Tuberville.
“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, the true supporter of our MAGA agenda!” the president tweeted on Friday.
He also linked to a campaign advert from Mr Tuberville, which excoriated Mr Sessions as a “DC insider”, a “total disaster” and an “embarrassment to Alabama”.
Later that evening, the former attorney general, who has mostly kept his silence despite years of taunts from the White House, hit back and tweeted directly at Mr Trump.
“Look, I know your anger, but recusal was required by law. I did my duty & you're damn fortunate I did. It protected the rule of law & resulted in your exoneration. Your personal feelings don't dictate who Alabama picks as their senator, the people of Alabama do.”
The winner of the Republican primary, which has been delayed until 14 July because of the coronavirus pandemic, will face off in November’s election against Democrat Doug Jones, who snatched the Senate seat in 2017 after Mr Sessions resigned.
Despite Mr Trump’s relentless attacks on him, Mr Sessions has continued to argue he will best carry out the president’s agenda if re-elected.
“Tuberville's a coward who is rightly too afraid to debate me. He says you're wrong on China & trade. He wants to bring in even more foreign workers to take American jobs. That's not your agenda and it's not mine or Alabama's,” he also tweeted on Friday.
Before he became attorney general, Mr Sessions had long been one of the Senate’s most hardline anti-immigration hawks. He was nominated to become a federal judge by Ronald Reagan in 1986 but was rejected by the Senate Judiciary Committee, only the second time in half a century senators had done so.