President Donald Trump mocked Rep. Adam Schiff, the California Democrat who is expected to become chairman of the House Intelligence Committee, in a tweet on Sunday, calling him “little Adam Schitt.”

“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!” Trump tweeted around 1 p.m.

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

About 30 minutes after the president’s tweet, Schiff tweeted:

Wow, Mr. President, that’s a good one. Was that like your answers to Mr. Mueller’s questions, or did you write this one yourself? https://t.co/Yd27sayt7C — Adam Schiff (@RepAdamSchiff) November 18, 2018

Schiff appeared earlier Sunday on ABC’s This Week, vowing to challenge President Trump’s appointment of acting Attorney General Matthew Whitaker.

“I think the appointment is unconstitutional. He is clearly a principle officer and the fact that he is a temporary principle officer doesn’t mean that that is any less subject to Senate confirmation,” he said.

The Vacancies Reform Act of 1998 allows for someone to serve in an acting capacity for 210 days as long as that official served in in the same department in a fairly high-ranking position for at least 90 days. Schiff argued there are special rules at the Justice Department to determine who becomes acting attorney general.

He also threatened to “expose” whether Whitaker has had any involvement in Special Counsel Robert Mueller’s investigation of the Trump campaign.

Before joining the Justice Department as former Attorney General Jeff Sessions’s chief of staff in 2017, Whitaker wrote op-eds and appeared on TV discussing his views about the investigation. He has said the investigation should not investigate the president’s finances that are unrelated to the question of whether there was collusion with the Russians.

“We will expose any involvement he has in it. He needs to know that if he takes any action to curb what Mr. Mueller does, we’re going to find out about it. We’re going to expose it. And I would certainly call on my colleagues right now to avoid the constitutional crisis, take action now, speak out against this appointment,” Schiff said.

It is not the first time Trump has trolled Schiff with a nickname. He has previously called him “Liddle Adam Schiff.”