President Donald Trump bestowed a juvenile nickname on Rep. Adam Schiff on Sunday, but the congressman responded with a barb of his own.

The president and the incoming House Intelligence Committee chairman taunted each other on Twitter, a preview of the coming months, since the rivalry is sure to intensify when Schiff resurrects the committee’s Russia investigation after Democrats assume control of the House of Representatives in January.

Trump lashed out at Schiff in a tweet, dubbing him “little Adam Schitt (D-CA).” He also diverted criticism of his appointment of acting Attorney General Matt Whitaker by questioning the appointment of special counsel Robert S. Mueller III. 

So funny to see little Adam Schitt (D-CA) talking about the fact that Acting Attorney General Matt Whitaker was not approved by the Senate, but not mentioning the fact that Bob Mueller (who is highly conflicted) was not approved by the Senate!

— Donald J. Trump (@realDonaldTrump) November 18, 2018

“Wow, Mr. President. That’s a good one,” Schiff responded. “Was that like your answers to Mr. Mueller’s questions, or did you write this one yourself?”

Trump instigated the spat minutes after Schiff called the appointment of Whitaker “unconstitutional” in a television appearance Sunday morning.

“He was chosen for the purpose of interfering with the Mueller investigation,” Schiff said on ABC. “He auditioned for the part by going on TV and saying he could hobble the investigation and ethically, he should have nothing to do with the investigation.”

The elevation of Whitaker — a Trump loyalist who has repeatedly criticized the Mueller probe — has angered Democrats.

Watch: Trump Says He Doesn’t Know Matt Whitaker

The appointment also touched off a legal dispute about the propriety of the White House sidestepping the “advise and consent” role conferred to Senate by the Constitution. The White House also overrode the Justice Department’s own statutes about the rules of succession in tapping Whitaker.

In the same television appearance, Schiff issued a warning to Whitaker that if he should interfere with the Mueller investigation, the Intelligence Committee is “going to find out about it. We’re going to expose it.”

Trump told reporters on Friday that he had finished answering Mueller’s questions — by himself — about possible ties between his presidential campaign and Russia, which Schiff referenced in his tweet.

“I’ve answered them very easily,” Trump said.