PALM BEACH, F.L. (CBSNewYork/AP) — President Donald Trump is accusing former President Barack Obama of having Trump’s telephones “wire tapped” during last year’s election, but Trump isn’t offering any evidence or saying what prompted the allegation.

Trump says in a series of tweets that he “just found out that Obama had my ‘wires tapped’ in Trump Tower.”

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The president went on to compare the alleged wiretapping to the infamous Watergate scandal that embroiled President Richard Nixon’s administration, ultimately ending in his 1974 resignation from office.

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

“As part of that practice, neither President Obama nor any White House official ever ordered surveillance on any U.S. citizen,” Lewis said, adding that “any suggestion otherwise is simply false.”

The White House did not immediately reply to inquiries about what prompted the president’s tweets.

Trump’s tweets came days after revelations that Attorney General Jeff Sessions, during his Senate confirmation hearing, didn’t disclose his own campaign-season contacts with Russia’s ambassador to the United States. Sessions, a U.S. senator at the time, was Trump’s earliest Senate supporter.

Trump’s opening tweet Saturday mentioned Sessions and claimed the first meeting Sessions had with the Russian diplomat was “set up by the Obama Administration under education program for 100 Ambs …”

U.S. intelligence agencies have concluded that Russia interfered in the campaign with the goal of helping elect Trump over Democrat Hillary Clinton — findings that Trump has dismissed. The FBI has investigated Trump associates’ ties to Russian officials. Congress is also investigating.

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Trump has blamed Democrats for leaks of information about the investigation and the contacts.

It was unclear what prompted Trump’s new charge. The president often tweets about reports he reads on blogs and conservative-leaning websites.

The allegations may be related to anonymously sourced reports in British media and blogs, and on conservative-leaning U.S. websites, including Breitbart News. Those reports claimed that U.S. officials had obtained a warrant under the Foreign Intelligence Surveillance Act to review contacts between computers at a Russian bank and Trump’s New York headquarters.

“It would be the biggest political scandal since Watergate,” Sen. Lindsey Graham, R-SC, said.

At a raucous town hall event, Graham said illegally wiretapping Trump would be troubling, but said it would be equally concerning if Trump’s phones were legally surveyed, which could mean there had been evidence of activity between the Trump campaign and foreign governments.

“It’s my job as a United States senator to get to the bottom of this, and I promise I will,” he told the crowd.

Meanwhile, Sessions traveled to Florida to have dinner with the president.

Trump is expected to sign an updated executive order banning travel from certain Middle Eastern and African countries early next week.

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(© Copyright 2017 CBS Broadcasting Inc. All Rights Reserved. The Associated Press contributed to this report.)