Sen. Claire McCaskill Claire Conner McCaskillMomentum growing among Republicans for Supreme Court vote before Election Day Democratic-linked group runs ads in Kansas GOP Senate primary Trump mocked for low attendance at rally MORE's (D-Mo.) reelection campaign said a staffer with a right-wing media outlet posing as a Democratic campaign staffer had access to Missouri voter roll data during the course of the campaign.

McCaskill's campaign wrote to Attorney General Josh Hawley (R), also her November general election opponent, demanding an investigation into whether a former McCaskill campaign staffer secretly employed by Project Veritas broke state law in the course of making undercover videos targeting McCaskill's staffers, according to The Kansas City Star.

Using a fake name, the Project Veritas employee released a series of undercover videos targeting McCaskill's campaign, including one in which a McCaskill staffer appears to say that voters "can't know" that the Democratic senator aligns politically with former President Obama.

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"The truth about her partisanship and party-line liberalism is revealed more every day," Hawley said in a statement following the video's release.

Hawley, who is in charge of the state's elections as attorney general, has so far refused to investigate Project Veritas over the videos. McCaskill accused Hawley of being directly involved with an effort to plant a staffer on her campaign.

"It is startling that Josh Hawley would be part of fraudulently embedding somebody in my campaign," the senator told a local news station, according to CBS News.

Hawley denied the accusations on Wednesday, and demanded that McCaskill provide evidence for her claims.

"Senator, accusing people of crimes is a serious thing," Hawley wrote on Twitter. "If you have evidence of a crime, please come forward with it immediately. Otherwise, please stop politicizing the legal process for your re-election."

Senator, accusing people of crimes is a serious thing. If you have evidence of a crime, please come forward with it immediately. Otherwise, please stop politicizing the legal process for your reelection. #MOsen — Josh Hawley (@HawleyMO) October 17, 2018

Missouri's Senate race is considered one of the tightest races in the country. A RealClearPolitics average of polls shows Hawley with an average lead of 0.5 percentage points over his opponent, while election forecasting site FiveThirtyEight gives McCaskill a 58 percent chance of retaining her seat.