PALM BEACH, Florida - US President Donald Trump is accusing then-president Barack Obama of ordering his telephones tapped during last year's elections, offering no evidence while invoking politically charged references to Watergate, Nixon and McCarthyism.

An Obama spokesman responded that the assertion against the former president was "simply false." Trump's claim also drew bipartisan rebukes from Democrats and Republicans alike.

In a series of morning tweets on Saturday, Trump suggested Obama was behind a politically motivated plot to upend his campaign. He compared the alleged events to "Nixon/Watergate" and "McCarthyism!" And he called Obama a "Bad (or sick) guy."

After Trump's well-received speech to Congress on Tuesday, the tweets reflected the president's growing frustration with the swirling allegations about his advisers' ties to Russia, which are under FBI investigation, and his team's inability to overcome them. Trump lashed out at his senior team during an Oval Office meeting on Friday, according to one White House official.

The White House did not respond to questions about what prompted the president's accusations that Obama had tapped his phones. Presidents cannot legally order wiretaps against US citizens. Obtaining wiretaps would require officials at the Justice Department to seek permission from the Foreign Intelligence Surveillance Court, which is shrouded in secrecy.

Trump said on Saturday morning he had "just found out" the information, though it was unclear whether he was referring to a briefing, a conversation or a media report. The president has in the past tweeted about unsubstantiated and provocative reports he reads on blogs or conservative websites.

The morning tweets stand out, even for the perpetually piqued Trump, given the gravity of the charge and the strikingly 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.

In his morning tweets, Trump said the wiretapping occurred in October at Trump Tower, the New York skyscraper where he ran his campaign and transition. He also maintains a residence there.

"How low has President Obama gone to tapp my phones during the very sacred election process. This is Nixon/Watergate. Bad (or sick) guy!" he tweeted, misspelling 'tap.'

Obama spokesman Kevin Lewis said a "cardinal rule" of the Obama administration was that no White House official ever interfered in Justice Department investigations, which are supposed to be conducted free of political influence.