Former Gov. Martin O’Malley (D-Md.) said Democrats are making a "big mistake" if they let front-runner Hillary Clinton Hillary Diane Rodham ClintonJoe Biden looks to expand election battleground into Trump country Biden leads Trump by 12 points among Catholic voters: poll The Hill's Campaign Report: Biden goes on offense MORE coast to the party's nomination without being tested.

“I think it’s a big mistake for us as a party to circle the wagons around the inevitable front-runner,” O'Malley, a rival contender for the 2016 Democratic nomination, told reporters in Manchester, N.H., according to CNN.

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O'Malley said the party needs a robust primary to ensure they have the strongest candidate in the general election.

The underdog has been pressing the Democratic National Committee to hold more than the planned six primary debates, accusing party leaders of trying to limit competition.

“Until we start having debates, our party’s going to be defined and branded by questions like: What did Secretary Clinton know, when did she know it, and when will the FBI conclude its investigation?” O’Malley said, citing controversy over Clinton’s use of a private email server while at the State Department.

“That’s not a formula for success in the fall,” he added. “These are serious and legitimate questions and Hillary Clinton and her lawyers will have to answer them.”

O’Malley also argued that he is more independent of wealthy special interests than Clinton.

“The Clintons are probably the most formidable fundraising couple that any republic in the history of the planet has ever created,” he said of Hillary Clinton and her husband, former President Bill Clinton.

“I am not the candidate awash in big Wall Street money,” O’Malley added. “That is not my niche.”

O’Malley’s remarks come as Clinton struggles with eroding voter trust following public revelations about her personal email server.

A new Quinnipiac University poll released on Thursday said that 61 percent of all voters believe that she is neither honest nor trustworthy.

O’Malley, though, defended Clinton's integrity.

“She’s never said anything to me that I could not rely on,” he said. “And in all of my personal dealings with her, I’ve found her to be forthright and honest.”

Clinton turned over her email server and a backup thumb drive to Justice Department investigators earlier this month.

Clinton has insisted she used the server only for convenience while secretary of State and denied any wrongdoing.

Critics counter that her use of the device prevented accountability and also jeopardized sensitive national intelligence she may have handled.

This story was updated at 1:38 p.m.