The president of the United States on Saturday accused his predecessor of wiretapping his phones in Trump Tower during the campaign, saying the alleged activity was comparable to the Watergate case.

Donald Trump leveled the startling accusations against Barack Obama in a series of early morning tweets, but offered no evidence to support his claims.

“How low has President Obama gone to tapp (sic) my phones during the very sacred election process. This is Nixon/Watergate. Bad (or sick) guy!” Trump wrote on Twitter.

He compared the purported actions to Watergate, the wiretapping scandal that forced then-President Richard Nixon to resign in disgrace in 1974, and to McCarthyism, right-wing US Sen. Joseph McCarthy’s notorious anti-communist crusade in the 1950s.

Among other tweets Saturday morning, Trump also said Attorney General Jeff Sessions’ first meeting with Russian Ambassador Sergey Kislyak in 2016 had been set up by the Obama administration.

Sessions is currently under fire for having met with the Russian ambassador during the presidential campaign even though he said at his confirmation hearing that he had had no communications with Russian officials.

Amid the controversy, Sessions earlier this week recused himself from any probe into charges that Russia interfered in the 2016 presidential election.

Trump also tweeted early Saturday that Kislyak visited the White House 22 times during Obama’s two terms in office, including on four occasions in 2016 alone.

Obama responds to allegations:

Former US President Barack Obama replied Saturday to his successor in the White House, Donald Trump, that he “never” ordered the wiretapping of a United States citizen,

“Neither President Obama nor any White House official ever ordered surveillance on any U.S. citizen,” said Kevin Lewis, spokesman for the former president in a brief statement.

Lewis added that “a cardinal rule of the Obama administration was that no White House official ever interfered with any independent investigation led by the Department of Justice.”

“Any suggestion otherwise is simply false,” the statement by Obama’s spokesman said.