Washington (CNN) President Donald Trump alleged Tuesday — without providing any evidence — that special counsel Robert Mueller's investigation will meddle in the midterm elections to benefit Democrats.

Trump's claim is his latest attack on the credibility of the Russia investigation as being politically motivated, though it's a significant new step in his attacks on what is intended to be an independent probe working to get to the bottom of Russia's efforts to interfere in the 2016 election.

"The 13 Angry Democrats (plus people who worked 8 years for Obama) working on the rigged Russia Witch Hunt, will be MEDDLING with the mid-term elections, especially now that Republicans (stay tough!) are taking the lead in Polls," Trump tweeted. "There was no Collusion, except by the Democrats!"

The 13 Angry Democrats (plus people who worked 8 years for Obama) working on the rigged Russia Witch Hunt, will be MEDDLING with the mid-term elections, especially now that Republicans (stay tough!) are taking the lead in Polls. There was no Collusion, except by the Democrats! — Donald J. Trump (@realDonaldTrump) May 29, 2018

Trump's use of the word "rigged" invokes a line he frequently employed in 2016 -- often when he was trailing Hillary Clinton in the polls -- to raise doubts about the election outcome. At the time, he appeared to be suggesting that the election would be out of the hands of voters.

Although CNN has reported that several members of Mueller's team have donated to Democrats, Russia's meddling in the 2016 election has also been the subject of several Republican-led congressional inquiries. Mueller is a Republican who was appointed as FBI director by President George W. Bush, and the man who appointed him as special counsel, Deputy Attorney General Rod Rosenstein, was appointed by Trump and is also a registered Republican.

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