President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE accused 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 on Saturday of hurting his and Republican candidates' approval ratings, again characterizing the special counsel's investigation as a "witch hunt."

The president tweeted Saturday afternoon that his approval ratings, which have hovered below 50 percent for weeks in most polls, and those of Republican candidates around the country would be higher if not for Mueller's investigation into possible ties between his campaign and Russia in the 2016 presidential election.

"While my (our) poll numbers are good, with the Economy being the best ever, if it weren’t for the Rigged Russian Witch Hunt, they would be 25 points higher!" Trump said.

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"Highly conflicted Bob Mueller & the 17 Angry Democrats are using this Phony issue to hurt us in the Midterms. No Collusion!" he added.

While my (our) poll numbers are good, with the Economy being the best ever, if it weren’t for the Rigged Russian Witch Hunt, they would be 25 points higher! Highly conflicted Bob Mueller & the 17 Angry Democrats are using this Phony issue to hurt us in the Midterms. No Collusion! — Donald J. Trump (@realDonaldTrump) September 15, 2018

Trump's tweet comes one day after Trump's former campaign chairman Paul Manafort Paul John ManafortOur Constitution is under attack by Attorney General William Barr Bannon trial date set in alleged border wall scam Conspicuous by their absence from the Republican Convention MORE pleaded guilty Friday to two federal charges. In pleading guilty, Manafort agreed to cooperate with Mueller's team in its investigation.

As part of his cooperation, Manafort has agreed to submit to interviews with the special counsel, testify in any future cases and provide related documents.

Manafort, the fourth Trump associate to plead guilty in Mueller's investigation, was found guilty last month of tax and bank fraud charges in a Virginia court and faced another trial in Washington, D.C., this month.

What Manafort's plea agreement means for Mueller's probe is yet unknown, but his cooperation could be significant for Mueller's investigation given his work on the Trump campaign.

Trump's tweet also followed a series of endorsements for Republican candidates across the country, including several candidates facing tight races in November like Nevada congressional candidate Danny Tarkanian (R) and Texas Rep. Pete Sessions Peter Anderson SessionsThe Hill's Campaign Report: New polls show Biden leading by landslide margins The Hill's Morning Report - Presented by Argentum - In Rose Garden, Trump launches anti-Biden screed Pete Sessions wins GOP runoff in comeback bid MORE (R).

Republicans hope to stave off a "blue wave" in November's midterm elections, in which Democrats hope to regain control of the House.

Trump's approval rating sits at 41 percent in a RealClearPolitics average of polls, while Republicans have lagged behind Democrats on generic ballot polls for control of the House from single- to double-digits since the beginning of the year.

Democrats' lead on an American Barometer poll commissioned by Hill.TV dropped by half this month to 5 percentage points after previously showing an 11-point lead last month.