The US president made the claim, without any supporting evidence, in a tweet following and earlier post describing the special counsel investigation into potential ties between the Trump campaign and Russian Federation, "the greatest witch hunt in American history".
The suggestion seems meant to undercut the work by special counsel Robert Mueller, who is investigating Russian interference in the 2016 election.
Conservative media figures from Wall Street Journal columnist Kimberly Strassel to radio host Rush Limbaugh have spent the last few days raising alarms about what they claim was an Federal Bureau of Investigation informant dispatched to "spy" on the Trump campaign.
Mueller's latest public moves have involved charging Trump's former campaign chairman Paul Manafort of money laundering and tax fraud from before his time working with Trump's campaign.
The Watergate scandal in the early 1970s occurred following a break-in by five men at Democratic Party headquarters at the Watergate building in Washington and subsequent attempts by the administration of President Richard Nixon to hide its involvement.More news: Simeone uses Atletico's EL win to try and force Griezmann's hand
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Former New York City Mayor Rudy Giuliani told Fox News host Laura Ingraham Wednesday on "The Ingraham Angle" that it was high time for Mueller to bring the probe "to a close" because it's "not good for the American people" to prolong it further.
The attacks on Mueller, a Republican and former FBI director, were expected to continue as the investigation entered its second year, according to Giuliani.
He said Mr Trump would only sit down with Mr Mueller if "we feel there's a way to shorten this thing".
He said the legal team was "pretty comfortable, in the circumstances of this case, that they wouldn't be able to subpoena him personally". "It's about time to say, 'Enough, we've tortured this president enough'". "We've tortured this president enough", he added, describing the investigation as being "like a big weight" on the president's back. "Maybe the special counsel never should have been appointed".