US President Donald Trump and Republican Senator Bob Corker on Sunday traded barbed tweets in an unusually harsh exchange between a president and a senator from his own party.
Trump began by stating that the retiring Tennessee senator had "begged" him for an endorsement for his re-election campaign.
The president tweeted he turned him down and that Corker then decided not to run again because he couldn't win without Trump's backing.
The president also contended that Corker "didn't have the guts to run" for a third term.
Corker responded with a jab of his own.
He tweeted: "It's a shame the White House has become an adult day care center. Someone obviously missed their shift this morning."
A spokesman for Corker later in the day disputed a key Trump assertion, saying that the president in fact told the senator he would have endorsed him for another term.
And The Associated Press has reported that Trump and Corker met privately at the White House last month, and that the president urged the senator to run.
In his tweets on Sunday, Trump also said he now expected Corker "to be a negative voice and stand in the way" of the White House's agenda, and accused him of being "largely responsible" for the landmark Iran nuclear deal.
But the Obama administration negotiated the international agreement, and Corker tried to require that President Barack Obama submit it to Congress for approval.
The two-term senator, who is also the Senate Foreign Relations Committee chairman, has taken a critical line toward the White House, and the president's leadership.
He recently said that the Pentagon chief, the secretary of state and Trump's own chief of staff were "those people that help separate our country from chaos".
The fact that the senator is not running for re-election gives him even more elbow room to say what he wants and to vote how he pleases over the next 15 months as Trump and Republican leaders struggle to get their agenda on track.