Donald Trump’s new acting attorney general is coming under pressure from all sides to keep his hands off the Mueller investigation into Russian interference in the US presidential election.

Adam Schiff, the prominent Democrat who will take control of the House intelligence committee in January, called the appointment of Matthew Whitaker “unconstitutional” and vowed to expose him if he attempts to thwart Robert Mueller’s investigation.

And Republican senators also lined up on Sunday to say Mueller must be allowed to continue unimpeded as he considers whether there was any collusion between the Trump campaign and Russia during the 2016 election.

Trump has renewed his virulent attacks on Mueller in recent days after forcing the attorney general, Jeff Sessions, to resign and replacing him with Whitaker.

The post is supposed to be filled by someone who goes through a Senate nomination process, but Trump installed Whitaker as acting attorney general and gave him oversight of the Mueller investigation.

Whitaker, who was chief of staff to Sessions, has been a vocal critic of Mueller and has appeared frequently on cable news channels setting out how the inquiry could be thwarted and starved of funds.

Schiff said: “It’s a flawed appointment, but the biggest flaw from my point of view is that he was chosen for the purpose of interfering with the Mueller investigation. He auditioned for the part by going on TV and saying he could hobble the investigation.”

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Schiff told ABC’s This Week: “He should have absolutely nothing to do with the investigation,” adding: “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 are going to find out about it, we are going to expose it.”

The debate later descended into farce on social media, as Trump derided Schiff on Twitter as “little Adam Schitt”, to which Schiff tweeted back: “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?”

Adam Schiff (@RepAdamSchiff) 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

The Republican senator Roy Blunt of Missouri said he had confidence in the Mueller investigation and joined calls for Trump to nominate a permanent attorney general who would go through the Senate process.

And Lindsey Graham, Republican senator for South Carolina, said the Senate should vote on a bipartisan bill, which he co-wrote, that would protect Mueller – a move being blocked by the Senate leader, Mitch McConnell.

Graham told NBC’s Meet the Press, however, that he had met Whitaker, had been impressed by his professionalism and did not expect him to interfere with Mueller. Whitaker told him he would not try to stop the investigation or cut off funding from the Department of Justice.

Graham said: “It would be a disaster for the Republican party for the Mueller investigation to be terminated, or shut down, or played with. He’s close to the end, let’s see what he finds.

“I can understand why the president doesn’t like the investigation, he feels wronged by it,” but he added: “Most Republicans on Capitol Hill, and Democrats, believe that Mr Mueller should be allowed to do his job.”

Trump said on Friday and Saturday that he had completed written answers for Mueller.

He has not agreed to Mueller’s request for a sit-down interview, however.

In a Fox interview broadcast on Sunday, Trump said he “would not get involved” if Whitaker decided to curtail Mueller’s investigation. “It’s going to be up to him,” he said and added: “I really believe he’s going to do what’s right.”