President Trump’s feud with a retiring GOP senator became heated on Sunday when the two exchanged increasingly personal insults on Twitter.

The back and forth demonstrated a deteriorating relationship between two Republicans who once considered running on the same presidential ticket.

The clash may also herald an impasse for the GOP’s legislative push on tax reform, due to Sen. Bob Corker Robert (Bob) Phillips CorkerHas Congress captured Russia policy? Tennessee primary battle turns nasty for Republicans Cheney clashes with Trump MORE’s (R-Tenn.) key vote.

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The war of words escalated early Sunday when Trump in a series of tweets claimed Corker sought the president’s endorsement for his reelection bid, which Trump said he declined.

“Senator Bob Corker ‘begged’ me to endorse him for re-election in Tennessee. I said ‘NO’ and he dropped out (said he could not win without.....my endorsement),” Trump said.

But Corker's chief of staff, Todd Womack, said Sunday that Trump pushed the Tennessee senator to run again, even offering his endorsement should Corker decide to do so.

"The president called Senator Corker on Monday afternoon and asked him to reconsider his decision not to seek reelection and reaffirmed that he would have endorsed him, as he has said many times," Womack told The Hill.

“He also wanted to be Secretary of State, I said ‘NO THANKS.’ He is also largely responsible for the horrendous Iran Deal!” Trump continued.

The president in his Sunday morning attack on the senator also accused Corker of not having “the guts to run” for reelection.

“...Hence, I would fully expect Corker to be a negative voice and stand in the way of our great agenda. Didn't have the guts to run!” Trump added.

Corker quickly fired back, describing the White House as “an adult day care center.”

“It's a shame the White House has become an adult day care center,” he said. “Someone obviously missed their shift this morning.”

Later on Sunday, Trump doubled down, this time explicitly blaming Corker for the Iran nuclear deal, an agreement negotiated between the United States, Iran, and international powers that provided Tehran with sanctions relief in exchange for curbs on its nuclear program.

"Bob Corker gave us the Iran Deal, & that's about it. We need HealthCare, we need Tax Cuts/Reform, we need people that can get the job done!" Trump tweeted.

Then, in a New York Times interview published on Sunday, Corker said Trump was treating the presidency like "a reality show" and that his threats against other countries could put the U.S. "on the path to World War III."

Corker also chairs the Senate Foreign Relations Committee and votes on the Senate Budget Committee. He has been a stalwart GOP voice on international affairs and national security.

He announced last month that he will not seek reelection to the Senate once his term ends in 2018.

The Tennessee lawmaker’s decision opens up the freedom for him to be more vocal in his criticisms of the administration, in part because he does not have to worry about alienating the base of Republican voters loyal to Trump.

While the attacks reached more personal levels over the weekend, it’s not the first time Trump and Corker have exchanged words.

The president in August claimed Tennessee is “not happy” with their senator and that Corker had asked his advice about reelection. Trump at the time was responding to Corker’s criticisms of the president’s response to the racially-motivated violence at a rally in Charlottesville, Va.

Corker recently made another veiled dig at Trump, saying key Cabinet officials like Secretary of State Rex Tillerson Rex Wayne TillersonGary Cohn: 'I haven't made up my mind' on vote for president in November Kushner says 'Alice in Wonderland' describes Trump presidency: Woodward book Conspicuous by their absence from the Republican Convention MORE and Secretary of Defense James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE, in addition to chief of staff John Kelly John Francis KellyMORE, “help separate our country from chaos.”

The three officials “work well together to make sure that the policies that we put forth around the world are sound and coherent, where other people within the administration, in my belief, are not,” Corker said last week.

White House press secretary Sarah Huckabee Sanders fired back that Trump is "the one that's keeping the world from chaos."

The growing hostility stands in stark contrast to the 2016 campaign, when Corker was under consideration to be Trump’s running mate. He was also thought to be a potential candidate for secretary of State during the transition.

Despite Corker’s impending departure from Congress, his committee assignments still present a possible hurdle to Trump, who is reportedly planning to de-certify the Iran nuclear deal. Such a move would give Congress 60 days to reimpose sanctions on Tehran.

Corker also sits on the Senate Budget Committee, which last week moved a budget resolution that will help the GOP’s effort to pass tax reform legislation without Democrats.

The Tennessee senator has emerged as one of the top skeptics of the Republican tax reform framework Trump rolled out last month.

“I want it to be pro-growth, and I want it to be permanent,” said Corker, who promised not to cast a vote for “one penny’s worth of deficits" in a new tax plan.

— Updated at 3:03 p.m.