A Democratic group has launched a six-figure ad campaign targeting five vulnerable Republican senators over the GOP's reluctance to call for witnesses as part of the ongoing Senate impeachment trial.

Majority Forward, a nonprofit affiliated with the Democratic super PAC Senate Majority, will air statewide ads targeting Republican Sens. Martha McSally Martha Elizabeth McSallyMcConnell locks down key GOP votes in Supreme Court fight Will Republicans' rank hypocrisy hinder their rush to replace Ginsburg? Trump supporters chant 'Fill that seat' at North Carolina rally MORE (Ariz.), Cory Gardner Cory Scott GardnerMcConnell locks down key GOP votes in Supreme Court fight Will Republicans' rank hypocrisy hinder their rush to replace Ginsburg? Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 MORE (Colo.), Susan Collins Susan Margaret CollinsMcConnell locks down key GOP votes in Supreme Court fight Most Americans think winner of election should pick next Supreme Court justice: poll Murkowski: Supreme Court nominee should not be taken up before election MORE (Maine), Joni Ernst Joni Kay ErnstPoll shows tight Senate race in Iowa Will Republicans' rank hypocrisy hinder their rush to replace Ginsburg? Momentum growing among Republicans for Supreme Court vote before Election Day MORE (Iowa) and Thom Tillis Thomas (Thom) Roland TillisMcConnell locks down key GOP votes in Supreme Court fight Will Republicans' rank hypocrisy hinder their rush to replace Ginsburg? Trump supporters chant 'Fill that seat' at North Carolina rally MORE (N.C.) in their respective states.

The 30-second ads “Oath” and “Rigged” hit the Republicans for not pushing for a vote to compel witnesses as part of the trial, the group announced Monday.

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The ads will run on digital and connected TV platforms, such as Hulu, through Friday. A spokesperson for Majority Forward said the campaign is a “sizable six-figure buy” split across the five states.

“Senate Republicans have broken their oath of impartiality and their promise to the American people by playing along with Mitch McConnell Addison (Mitch) Mitchell McConnellOcasio-Cortez to voters: Tell McConnell 'he is playing with fire' with Ginsburg's seat McConnell locks down key GOP votes in Supreme Court fight Video shows NYC subway station renamed after Ruth Bader Ginsburg MORE’s cover-up,” Senate Majority PAC President J.B. Poersch said in the announcement.

“By refusing to get the facts and demand a fair trial from the onset, Senate Republicans are putting party politics over principle. Our new ad campaign urges these vulnerable incumbents to do their jobs and demand a fair trial now,” Poersch added.

A vote on whether to allow witnesses is expected to take place later this week. If all Democrats vote in favor of the measure, four Republicans will need to join them for the resolution to pass.

Democrats have been pushing for witnesses as part of the Senate trial, arguing that by not allowing them, Republicans are blocking the opportunity for a fair trial.

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Some Republicans have signaled they may vote for witnesses, especially in the wake of a bombshell New York Times report on former national security adviser John Bolton John BoltonDiplomacy with China is good for America The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Pence lauds Harris as 'experienced debater'; Trump, Biden diverge over debate prep DOJ launches probe into Bolton book for possible classified information disclosures MORE’s upcoming book.

The Times reported on Sunday night that Bolton claims in his forthcoming memoir that President Trump Donald John TrumpBiden leads Trump by 36 points nationally among Latinos: poll Trump dismisses climate change role in fires, says Newsom needs to manage forest better Jimmy Kimmel hits Trump for rallies while hosting Emmy Awards MORE tied Ukraine aid to help with investigations into Democrats, including former Vice President Joe Biden Joe BidenBiden leads Trump by 36 points nationally among Latinos: poll GOP set to release controversial Biden report Can Donald Trump maintain new momentum until this November? MORE and his son Hunter Biden. The allegations are at the heart of the impeachment trial, and Bolton is one of several firsthand witnesses Democrats have been calling for.

Collins said Monday the reports about Bolton’s book “strengthen the case for witnesses and have prompted a number of conversations among my colleagues.”

Sen. Mitt Romney Willard (Mitt) Mitt RomneyGOP set to release controversial Biden report McConnell locks down key GOP votes in Supreme Court fight Will Republicans' rank hypocrisy hinder their rush to replace Ginsburg? MORE (R-Utah) similarly told reporters Monday that it is “increasingly likely” additional GOP senators will support calling Bolton.