WASHINGTON (Reuters) – U.S. President Donald Trump blamed Senator Bob Corker for the Iran nuclear deal on Sunday in a series of derisive Twitter posts that drew a sharp comeback from the lawmaker, an influential fellow Republican and onetime ally.

The senator, who announced his retirement last month, was a national security adviser to Trump during the 2016 presidential campaign and on Trump’s short list last year for both vice president and secretary of state.

More recently, he has criticized Trump, taking issue with the president’s response to a white supremacist demonstration in Charlottesville, Virginia, in August. Critics assailed Trump for saying rival protesters were also to blame for violence.

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

“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!”

An hour later Corker, chairman of the Senate Foreign Relations Committee, tweeted back: “It’s a shame the White House has become an adult day care center. Someone obviously missed their shift this morning.”

Corker’s chief of staff, Todd Womack, said Trump called the senator last Monday, asked him to reconsider his decision not to seek re-election in 2018, “and reaffirmed that he would have endorsed him, as he has said many times.”

Trump is expected to disclose within days a plan to decertify the 2015 international nuclear agreement with Iran, putting its future in the hands of Congress, where Corker would play a central role in determining its fate.

Trump has long criticized the pact, a signature foreign policy achievement of Democratic former President Barack Obama in which Iran agreed to reduce its nuclear program in exchange for easing of international sanctions.

During the U.S. political fight over the agreement, which was opposed by every Republican in Congress, Corker co-wrote the law that required congressional approval for the deal and required the president to certify that Iran was complying every 90 days.

Some critics on the far right have blamed Corker for that measure, which they say helped push the pact through Congress.

The two-term senator’s retirement is a blow to a Republican Party struggling to balance divisions between mainstream and more populist wings.

Asked last week about reported tensions between Trump and Secretary of State Rex Tillerson, Corker responded with what was seen as a jab at Trump.

He described Tillerson, Secretary of Defense James Mattis, and White House Chief of Staff John Kelly as “those people that help separate our country from chaos.”

After Charlottesville, Corker said, “The president has not yet been able to demonstrate the stability, nor some of the competence, that he needs to demonstrate in order to be successful.”