The chairman of the House intelligence committee said on Sunday it would be a “mistake” if special counsel Robert Mueller does not get Donald Trump to testify before a grand jury assigned to weigh criminal charges in the ongoing investigations of Russia and the Trump campaign.

Representative Adam Schiff of California reopened the committee’s investigation of the Trump campaign when Democrats took control of the House of Representatives in January.

On Sunday, he told Chuck Todd of NBCs Meet the Press that it would be a misstep for Mueller to accept Trump’s testimony in writing.

“Ultimately it’s a mistake, because probably the best way to get the truth would be to put the president under oath,” Schiff said. “Because as he’s made plain in the past he feels it’s perfectly fine to lie to the public. After all he has said: ‘It’s not like I’m talking before a magistrate.’

“Well maybe he should talk before a magistrate.”

I’ve said all along that I don’t think Bob Mueller should rely on written answers Adam Schiff

Negotiations last year between Mueller and lawyers for Trump ended without an agreement by the president to testify. Instead, lawyers submitted written answers to prosecutors’ questions.

While Mueller could subpoena the president in an attempt to force Trump to testify, the White House would be expected to resist the effort. Such a subpoena, in addition, would likely require clearance by attorney general William Barr, a Trump appointee.

Trump lawyer Rudy Giuliani last year admitted he did not want the president to testify under oath, framing the likelihood of Trump lying to prosecutors as a “perjury trap” laid by Mueller.

Famously, Watergate reporter Bob Woodward concluded his 2018 book Fear: Trump in the White House by writing that one-time Trump lawyer John Dowd identified the president’s “one overriding problem” which he “could not bring himself to say to the president: ‘You’re a fucking liar.’”

Testimony under oath holds represents a clear legal hazard for Trump. Bill Clinton testified in 1998 before a grand jury convened by independent prosecutor Kenneth Starr. Articles of impeachment against Clinton accused him of making “perjurious, false, and misleading statements before the grand jury”.

But Schiff warned that without sworn testimony by Trump, the public is unlikely to learn the details of his business pursuits in Russia, his communications with Russian president Vladimir Putin and other matters.

“I’ve said all along that I don’t think Bob Mueller should rely on written answers,” Schiff said, adding: “Here you need to be able to ask follow-up questions in real time.”

Schiff also said he believed that Trump might still be hoping to build a tower in Moscow or to make a similar deal.

“It potentially explains the president’s bizarre affinity for Vladimir Putin,” he said, “and that is, that he stood to make more money from this transaction than any deal in his life and sought the Kremlin’s help to make it happen while concealing this from the public.

“That still may be an animating principle for the president, he may believe that when he leaves office that he still wants to build this tower. And while that may not be criminal it is nonetheless corrupt.”