US lawmakers from both parties say they've seen no proof to support the claim by Republican President Donald Trump that his predecessor Barack Obama had wiretapped him last year - adding pressure on Trump to explain or back off his repeated assertion.
Several Republicans last week urged Mr Trump to apologise for the allegations he made in a series of tweets on March 4.
The maelstrom also caused tension with key US allies and threatens to distract Republicans from campaign promises on health care and taxes.
"I don't know the basis for President Trump's assertion," US Senator Susan Collins, a Republican, said on NBC's Meet the Press on Sunday.
"I do believe he owes us that explanation."
Ms Collins said she supported Mr Trump as president, but she wouldn't side with him if he "misstated what the facts are".
FBI director James Comey is expected to be asked about Mr Trump's claims when he testifies at a rare public hearing on Monday about alleged Russian meddling in the 2016 presidential election. Russia has denied the assertion.
Representative Adam Schiff, the top Democrat on the House Intelligence Committee holding the hearing, called Mr Trump's claims "patently false" and said he expected Mr Comey to say as much on Monday.
The White House and Trump allies have sought to focus attention away from the controversies by calling for investigations of leaks to the news media.
Representative Devin Nunes, who leads the House intelligence panel, said leaks to reporters about former Trump national security adviser Michael Flynn were criminal and that his panel was probing whether other names were leaked.
Mr Nunes, despite being a Trump ally who served on the president-elect's transition team, has not taken Mr Trump's side on the wiretapping claim. On Fox, he said he did not believe it occurred.
Still, the White House has not backed down. The administration was forced to reassure key ally Britain after White House press secretary Sean Spicer repeated a Fox News analyst's claim that a British intelligence agency helped Mr Obama wiretap Mr Trump. The British government strongly denied it.
Senior Republican Representative Tom Cole told reporters on Friday that Mr Trump owed Obama an apology. Representatives Charlie Dent and Will Hurd, also Republicans, made similar comments.