PHOENIX — U.S. Sen. John McCain (R-Ariz.) demanded Tuesday that President Donald Trump release information he used to accuse his predecessor of wiretapping Trump’s New York City home and campaign headquarters.

“The president of the United States, who has stated categorically that Trump Tower was wiretapped, should come forward with the information that led him to that conclusion,” McCain told reporters.

Over the weekend, Trump tweeted multiple times that former President Barack Obama had ordered the wiretap on the Manhattan skyscraper.

Terrible! Just found out that Obama had my "wires tapped" in Trump Tower just before the victory. Nothing found. This is McCarthyism! — Donald J. Trump (@realDonaldTrump) March 4, 2017

How low has President Obama gone to tapp my phones during the very sacred election process. This is Nixon/Watergate. Bad (or sick) guy! — Donald J. Trump (@realDonaldTrump) March 4, 2017

McCain said the president’s allegations carry serious weight and he must let people know where he got his information so it can be investigated if need be.

“That’s a very serious charge against the previous president of the United States,” he said. “There needs to be some corroboration.”

White House aides defended Trump’s claims on Monday. When asked where Trump was getting his information from, White House Deputy Press Secretary Sarah Huckabee Sanders said the president “may have access to documents that I don’t know about.”

Likewise, Kellyanne Conway said that “credible news sources” suggested there was politically motivated activity during the campaign. But Conway also said Trump might have access to other information she and others don’t.

“He is the president of the United States,” Conway told Fox News’ “Fox & Friends.” ”He has information and intelligence that the rest of us do not.”

The Justice Department, not the president, would have the authority to conduct such surveillance, and officials have not confirmed any such action.

Through a spokesman, Obama said neither he nor any White House official had ever ordered surveillance on any U.S. citizen.

Obama’s top intelligence official, James Clapper, also said Trump’s claims were false, and a U.S. official said the FBI asked the Justice Department to rebut Trump’s assertions.

Josh Earnest, who was Obama’s White House press secretary, said presidents do not have authority to unilaterally order the wiretapping of American citizens, as Trump has alleged was done to him. FBI investigators and Justice Department officials must seek approval from a federal judge for such a step.

Earnest accused Trump of leveling the allegation to distract from the attention being given to the Russia issue.

Trump said in the tweets that he had “just found out” about being wiretapped. Unclear was whether he was referring to having learned through a briefing, a conversation or a media report. The president in the past has tweeted about unsubstantiated and provocative reports he reads on blogs or conservative websites.

The tweets stood out, given the gravity of the charge and the sharp personal attack on the former president. Trump spoke as recently as last month about how much he likes Obama and how much they get along, despite their differences.

The Associated Press contributed to this report.

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