Former Massachusetts Gov. William Weld William (Bill) WeldRalph Gants, chief justice of Massachusetts supreme court, dies at 65 The Hill's Campaign Report: Biden visits Kenosha | Trump's double-voting suggestion draws fire | Facebook clamps down on election ads Biden picks up endorsements from nearly 100 Republicans MORE (R) on Thursday joined top Democrats in calling for 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 to testify before Congress to "address the validity of his report," which will be released Thursday and will be partially redacted.

Weld, who has launched a 2020 primary challenge against 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, also questioned whether the report's conclusions were determined by Mueller or if they have been "altered."

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"Confidence in our leaders and in our institutions is at the heart of our democracy," Weld wrote in a statement posted to Twitter. "That confidence has been shaken. It is essential that Special Counsel Bob Mueller come before Congress and address the validity of his report that has been redacted and released. Is this the report he issued?"

"Are these the conclusions to which he came or has the report been altered in any way? Was he pressured to remove or change anything?" he continued.

It is essential that Special Counsel Bob Mueller come before Congress and address the validity of his report that has been redacted and released. Is this the report he issued? (2/6) — Gov. Bill Weld (@GovBillWeld) April 18, 2019

Are these the conclusions to which he came or has the report been altered in any way? Was he pressured to remove or change anything? (3/6) — Gov. Bill Weld (@GovBillWeld) April 18, 2019

Weld, who led the Criminal Division of the Justice Department under former President Reagan, added that he has known Mueller for "over 30 years" and has "every confidence in his ability and his integrity."

Mueller investigated Russian interference in the 2016 election. Prior to Thursday's release of his report, Attorney General William Barr Bill BarrHarris faces pivotal moment with Supreme Court battle Hillicon Valley: DOJ proposes tech liability shield reform to Congress | Treasury sanctions individuals, groups tied to Russian malign influence activities | House Republican introduces bill to set standards for self-driving cars McCarthy threatens motion to oust Pelosi if she moves forward with impeachment MORE offered a defense of Trump in a press conference, saying the report proves there was no collusion between the Trump campaign and Russia.

Ahead of Barr's press conference, House Speaker Nancy Pelosi Nancy PelosiHoyer: House should vote on COVID-19 aid — with or without a bipartisan deal Ruth Bader Ginsburg lies in repose at Supreme Court McCarthy threatens motion to oust Pelosi if she moves forward with impeachment MORE (D-Calif.) and Senate Minority Leader Charles Schumer Chuck SchumerCruz blocks amended resolution honoring Ginsburg over language about her dying wish Senate Democrats introduce legislation to probe politicization of pandemic response Schumer interrupted during live briefing by heckler: 'Stop lying to the people' MORE (D-N.Y.) called on Mueller to testify, citing their objections to Barr's handling of the long-awaited Mueller report release.