WASHINGTON  — The House intelligence committee is asking the Trump administration for evidence that the phones at Trump Tower were tapped during the campaign as its namesake has charged, a request reinforced Sunday by an influential Republican senator who says the president must either come up with the evidence or retract his claim.

“I think the president has one of two choices: either retract or to provide the information that the American people deserve, because, if his predecessor violated the law, President Obama violated the law, we have got a serious issue here, to say the least,” Sen. John McCain said.

President Donald Trump asserted in a tweet last week: “Terrible! Just found out that Obama had my ‘wires tapped’ in Trump Tower just before the victory. Nothing found. This is McCarthyism!” He continued the allegation against former President Barack Obama in other tweets but offered no evidence.

The request for evidence by Monday was made in a letter sent to the Justice Department by the House committee chairman, Rep. Devin Nunes, R-Calif., and the panel’s ranking Democrat, Rep. Adam Schiff, D-Calif., a senior congressional aide said Saturday. The aide wasn’t authorized to discuss the request by name and requested anonymity.

Obama’s director of national intelligence, James Clapper, has said that nothing matching Trump’s claims had taken place, but that has not quelled speculation that Trump’s communications were monitored by the Obama administration. Trump has asked Congress to investigate.