President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE asserted on Sunday that the special counsel investigation into possible collusion between his presidential campaign and Russia is "illegal" because it was ignited by a "fraudulent" opposition research dossier.

In a tweet, Trump repeated his frequent claim that the probe is a "Witch Hunt," and that special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE's team of investigators is made up of a number of political opponents intent on undermining his presidency.

"There is No Collusion!" Trump tweeted. "The Robert Mueller Rigged Witch Hunt, headed now by 17 (increased from 13, including an Obama White House lawyer) Angry Democrats, was started by a fraudulent Dossier, paid for by Crooked Hillary and the DNC. Therefore, the Witch Hunt is an illegal Scam!"

There is No Collusion! The Robert Mueller Rigged Witch Hunt, headed now by 17 (increased from 13, including an Obama White House lawyer) Angry Democrats, was started by a fraudulent Dossier, paid for by Crooked Hillary and the DNC. Therefore, the Witch Hunt is an illegal Scam! — Donald J. Trump (@realDonaldTrump) July 29, 2018

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The tweet was his latest attack on what has become one of his most frequent targets. Trump has repeatedly sought to cast the Russia investigation as a freewheeling expedition to gather dirt on his campaign and associates.

The tweet came just over a week after the Justice Department released documents related to secret surveillance warrants obtained on Carter Page, a former adviser to Trump's campaign.

Those documents cite allegations in a dossier compiled in 2016 by a former British intelligence agent that Page met with two senior Russian representatives during a trip to Moscow that year.

The research behind the dossier was funded, in part, by Hillary Clinton Hillary Diane Rodham ClintonFox News poll: Biden ahead of Trump in Nevada, Pennsylvania and Ohio Trump, Biden court Black business owners in final election sprint The power of incumbency: How Trump is using the Oval Office to win reelection MORE's presidential campaign.

Some Republicans have long argued that the application to obtain the surveillance warrant against Page relied too heavily on the dossier, which contains salacious and controversial claims that have not been verified.

But Democrats have pointed to the fact that the warrant application contains evidence against Page that is unrelated to the dossier, such as efforts by Moscow to recruit Americans as intelligence assets in 2013. Page was reportedly one of those targets.