Senator Bob Corker, a Republican from Tennessee, and President Trump were once friendly. In fact, at one time, Trump was even considering naming the chairman of the Senate Foreign Relations Committee his running mate. But the relationship has soured in recent days—naturally starting with a spat on Twitter.

After Corker announced that he would not be seeking re-election in 2018, Trump accused the Senator of not having “the guts to run,” and suggested that Corker had “begged” Trump to endorse his re-election. When Trump declined, Corker “dropped out” as he “could not win” without Trump’s endorsement.

Trump’s tirade went on to accuse Corker of wanting to be Secretary of State, (which Trump said “NO THANKS” to), and called Corker “largely responsible for the horrendous Iran Deal.” Corker hit back, saying that “it’s a shame the White House has become an adult day care center. Someone obviously missed their shift this morning.”

While Corker has denied Trump’s account, the dispute reveals a larger issue in the Republican party. Corker is the first senator to announce that he will retire ahead of next year’s primaries and has increasingly criticized the president.

And in an interview with the New York Times published following the Twitter spat on Sunday, Corker leveled even sharper criticism against Trump. He told the Times that Trump treats his office like “a reality show,” acting like “he’s doing ‘The Apprentice’ or something.” Corker continued that he knows “for a fact that every single day at the White House, it’s a situation of trying to contain him” and went so far as to suggest that Trump’s threats could put the U.S. “on the path to World War III.”

The White House reportedly did not respond to a request for comment.