President Donald Trump went after Democratic Rep. Adam Schiff on Twitter on Sunday, and called the California congressman “Schitt.”

Trump blasted Schiff, the likely incoming chairman of the House Intelligence Committee, over his remarks regarding acting Attorney General Matt Whitaker. The president criticized Schiff for noting that Whitaker was not confirmed by the Senate but not ensuring special counsel Robert Mueller received the same treatment.

“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 wrote.

The president’s tweet was reportedly up on Twitter for more than an hour after he posted it.

Shortly afterwards, Schiff fired back by writing: “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?”

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Schiff’s jab was a reference to Trump saying he would soon provide his written answers to Mueller’s questions on the Russia investigation.

Schiff said in an interview on ABC’s This Week on Sunday that Democrats — who now hold the majority in the House of Representatives — will fight the lawfulness of Trump’s appointment of Whitaker, which many legal experts have called potentially unconstitutional, come January.

“I think the appointment is unconstitutional,” Schiff said of Whitaker’s new role as head of the Justice Department.

Whitaker has been criticized by many Democratic lawmakers for his extreme willingness to defend Trump in the Russia probe. The former U.S. Attorney from Iowa has publicly stated that he believes he can undermine Mueller’s inquiry without violating any laws. Whitaker also previously worked for a company that was discovered to have defrauded clients.

Speaking on Fox News on Sunday, Trump said he believes Whitaker’s stance on the Mueller probe is “right.”

“What do you do when a person’s right?” Trump told Fox News’ Chris Wallace. “There is no collusion. He happened to be right. I mean, he said it. So if he said there is collusion, I’m supposed to be taking somebody that says there is? Because then I wouldn’t take him for two reasons, but the number one reason is the fact that he would have been wrong. If he said that there’s no collusion, he’s right.”