Fellow Republicans are pressing President Donald Trump to come clean about whether he has tapes of private conversations with former FBI director James Comey.
The request is a sign of escalating fallout from riveting testimony from Mr Comey last week of undue pressure from Mr Trump. Mr Trump has responded to Mr Comey's assertions by accusing him of lying.
Meanwhile, the Senate investigation into collusion with Russia or obstruction of justice is extending to a Trump Cabinet member.
Attorney General Jeff Sessions is in for sharp questioning on Tuesday by senators on the Senate intelligence committee. Whether that hearing will be public or closed is not yet known.
"I don't understand why the president just doesn't clear this matter up once and for all," said Senator Susan Collins, a member of that committee, referring to the existence of any recordings.
She described Mr Comey's testimony as "candid" and "thorough" and said she would support a subpoena if needed. Mr Trump "should voluntarily turn them over," Ms Collins said.
Senator James Lankford, also a member of that committee, agreed the panel needed to hear any tapes that exist. "We've obviously pressed the White House," he said.
Mr Trump's aides have dodged questions about whether conversations relevant to the Russia investigation have been recorded, and so has the president.
Pressed on the issue Friday, Mr Trump said "I'll tell you about that maybe sometime in the very near future."
Mr Lankford said Mr Sessions's testimony on Tuesday will help flesh out the truth of Mr Comey's allegations, including Mr Sessions's presence at the White House in February when Mr Trump asked to speak to Mr Comey alone.
Mr Comey alleges that Mr Trump then privately asked him to drop a probe into former national security adviser Michael Flynn's contacts with Russia.
Mr Comey also has said Mr Sessions did not respond when he complained he did not "want to get time alone with the president again".
The Justice Department has denied that, saying Mr Sessions stressed to Mr Comey the need to be careful about following appropriate policies.
"We want to be able to get his side of it," Mr Lankford said.
Senator Jack Reed said "there's a real question of the propriety" of Mr Sessions's involvement in Mr Comey's dismissal, because Mr Sessions had stepped aside from the federal investigation into contacts between Russia and the Trump campaign. Mr Comey was leading that probe.
Mr Reed said he also wants to know if Mr Sessions had more meetings with Russian officials as a Trump campaign adviser than have been disclosed.
Mr Trump on Sunday accused Mr Comey of "cowardly" leaks and predicted many more from him. "Totally illegal?" he asked in a tweet. "Very 'cowardly!'"
Several Republican politicians also criticised Mr Comey for disclosing memos he had written in the aftermath of his private conversations with Mr Trump, calling that action "inappropriate". But, added Mr Lankford "releasing his memos is not damaging to national security".
The New York City federal prosecutor who expected to remain on the job when Mr Trump took office but ended up being fired said he was made uncomfortable by one-on-one interactions with the president - just like Mr Comey was.
Preet Bharara told ABC's This Week that Mr Trump was trying to "cultivate some kind of relationship" with him when he called him twice before the inauguration to "shoot the breeze".
He said Mr Trump reached out to him again after the inauguration but he refused to call back, shortly before he was fired.
On Mr Comey's accusations that Mr Trump pressed him to drop the FBI investigation of Flynn, Mr Bharara said "no one knows right now whether there is a provable case of obstruction" of justice. But: "I think there's absolutely evidence to begin a case."