An Arizona Republican is calling on special counsel Robert Mueller to recuse himself from the investigation into possible ties between the Trump campaign and Russia in an op-ed published Wednesday.

Writing in USA Today, Rep. Andy Biggs (R-Ariz.) said Mueller's investigation has gone "far beyond the scope of his original charge."

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"Unfortunately, taxpayer resources are being used to scrutinize anyone and anything that fits into a preconceived notion that Russia colluded with the Trump campaign to 'fix the election,'" Biggs said.

"Mueller has demonstrated he is incapable of leading a focused, unbiased review of his initial assignment," he added. "His witch hunt must end."

Biggs attacked the special counsel over agents on his team critics say are biased against 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. One FBI agent, Peter Strzok, made headlines this month after text messages revealed he referred to the investigation as an "insurance policy" against Trump's election. Strzok was removed from his role in the investigation.

"We should be deeply concerned with Mueller’s unlimited reach and unvetted hires. The way Mueller has conducted and unilaterally expanded his investigation, the subordinates he has hired, the persistent leaks from his office, and his abuse of attorney-client privilege — would all suggest his preference for questionable tactics," Biggs wrote.

"He has repeatedly shown that he has no interest in ending his crusade. The credibility of the American justice system — and indeed, our representative republic — is at stake," he added.

Biggs, a freshman Republican lawmaker, has called for Mueller's recusal since June, which he notes in the piece.

Other Republicans have cast doubt on the integrity of Mueller's team, raising speculation over whether the president will order the firing of the special counsel.

The White House has repeatedly denied that Trump is considering firing Mueller or ending the special counsel investigation.