CHICAGO (CBS) — President Donald Trump took to Twitter Saturday to accuse former President Barack Obama of wiretapping phone calls made by Trump during the campaign.

But as CBS 2’s Brook Silva-Braga reports, Trump cited no evidence or source.

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

The President wrote that the wiretapping happened at Trump Tower, but offered no evidence.

I'd bet a good lawyer could make a great case out of the fact that President Obama was tapping my phones in October, just prior to Election! — Donald J. Trump (@realDonaldTrump) March 4, 2017

The tweets follow a story published on Friday on the conservative website “Breitbart,” which detailed similar claims of Obama wiretapping Trump.

A spokesman for the former President responded in part: “Neither President Obama nor any White House official ever ordered surveillance on any U.S. citizen. Any suggestion otherwise is simply false.”

Republican Senator Lindsey Graham said that “would be the biggest political scandal since Watergate.”

At a raucous town hall event, Senator Graham said illegally wiretapping Trump would be troubling. But he added that it would be equally concerning if Trump was legally surveilled. That could mean there had been evidence of activity between the Trump campaign and foreign governments.

“It’s my job, as United States Senator to get to the bottom of this, and I promise you I will,” Graham said.

Attorney General Jeff Sessions, who recently recused himself from the investigation of links between the Trump campaign and Russia, also traveled to Florida for dinner with the President.

Trump also tweeted Saturday about Arnold Schwarzenegger leaving the new Celebrity Apprentice, saying he was fired by the network for “pathetic ratings.”