Manchester, New Hampshire (CNN)Martin O'Malley took one of the hardest swings of any Democrat yet at Hillary Clinton on Thursday, saying the party shouldn't be "circling the wagons" around the former secretary of state and questioning her viability against Republicans.
O'Malley: Rallying around Clinton a 'big mistake'
The former Maryland governor -- struggling to climb out of low single digits in national Democratic primary polls -- said Clinton will continue to be dogged by her use of a personal email address on a private server during her tenure as America's top diplomat.
"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 told reporters in New Hampshire. "That's not a formula for success in the fall."
O'Malley went further than other Democratic presidential candidates have. Clinton's top-polling challenger, Vermont Sen. Bernie Sanders, has avoided direct intra-party attacks and instead trained his fire on Wall Street and Republicans.
He also criticized the Democratic National Committee for scheduling only six debates, saying those nationally-televised events are opportunities for the party to focus on big ideas, rather than Clinton's email drama.
"I think it's a big mistake for us as a party to circle the wagons around the inevitable frontrunner," O'Malley said Thursday.
The email controversy -- regularly given new life as the State Department releases new batches of Clinton's messages each month and a House panel prepares to call the former secretary of state in for a hearing -- has taken a toll on Clinton's trustworthiness, surveys show.
A new Quinnipiac University poll found that just 34% of registered voters find Clinton trustworthy -- while 61% say they do not.
The email troubles have given rise to questions among some Democrats about whether Clinton's viability in the 2016 general election is being damaged -- particularly with the growing prospect that Vice President Joe Biden could enter the Democratic primary.
Asked about her emails this week, Clinton showed more contrition than she had in recent campaign events -- dropping the jokes she'd made about Snapchat and wiping down her server with a cloth and acknowledging that using only a personal address "clearly wasn't the best choice."
"I know people have raised questions about my email use as secretary of state, and I understand why. I get it," she told reporters Wednesday in Iowa.
O'Malley, who endorsed Clinton over President Barack Obama in the 2008 Democratic presidential primary, said Thursday that he doesn't think "a resort to one of the older names is going to move us forward."
He also attempted to tie Clinton to Wall Street -- one of the most popular targets on the Democratic campaign trail this year.
"I am not the candidate awash in big Wall Street money. That is not my niche," he said. "The Clintons are probably the most formidable fundraising couple that any republic in the history of the planet has ever created."
Still, he offered Clinton some cover on questions of her trustworthiness, saying: "I know Secretary Clinton, and I supported her eight years ago. I have a great deal of admiration for her on a personal level and a professional level. She's never said anything to me that I could not rely on. And in all of my personal dealings with her, I've found her to be forthright and honest."
O'Malley had also criticized Clinton in an interview with the New Hampshire radio station WGIR earlier Thursday.
He called the email probe "very serious" and said that there are "legitimate questions" about whether she handled classified material on a non-government server.
"These are serious and legitimate questions and Hillary Clinton and her lawyers will have to answer them," he said.