Democratic presidential candidate Bernie Sanders speaks at an event sponsored by Institute of Politics at the University of Chicago recently. (Scott Olson/Getty Images)

CONCORD, N.H. — As he filed Thursday to run in New Hampshire’s Democratic contest for president, Sen. Bernie Sanders (I-Vt.) fielded a colorful question from someone crammed into the secretary of state’s office here: “You ready to kick some Republican butt, Bernie?”

“There’s some earlier butt we have to deal with,” the Vermont senator replied, in a less-than-graceful reference to the Democratic front-runner Hillary Rodham Clinton.

The remark, which prompted some nervous laughter in a room packed with reporters and supporters, comes as Clinton’s camp has already suggested that Sanders and his team are treating her in a sexist fashion — a notion Sanders has vigorously denied. Thursday’s comment seemed likely to provide additional fodder.

There had been controversy of another sort prior to Sanders’s filing Thursday, with some questioning whether the longest-serving independent in Congress would be eligible to run as a Democrat in the nation’s first primary state.

That issue was not a hindrance Thursday, though Sanders’s standing could still be challenged. His aides have downplayed that as a concern.

“I am running as a Democrat, obviously,” Sanders told reporters. “I am a Democrat now.”

Raymond Buckley, chairman of the New Hampshire Democratic Party, accompanied Sanders to the statehouse in a show of support for his eligibility, which is also backed by the Democratic National Committee.

Sanders was greeted by an enthusiastic crowd of more than 700 people when he emerged from the statehouse for a post-filing rally, where he railed against a "rigged" economic system and a "corrupt" political system.

Clinton is scheduled to file as a candidate Monday in New Hampshire, where recent polls have shown a tight race with Sanders.

A third Democratic hopeful, former Maryland governor Martin O’Malley, filed Wednesday.

The Clinton and Sanders camps, meanwhile, sparred Thursday over whether Sanders was engaging in “personal attacks” regarding Clinton’s use of a private email server while secretary of state.

In perhaps the most memorable line of the first Democratic debate in Las Vegas, Sanders turned to Clinton and said, “The American people are sick and tired of hearing about your damn emails.”

On Wednesday, the Wall Street Journal published an interview with Sanders in which he said there are "valid questions" about Clinton's emails and that an FBI investigation should "proceed unimpeded."

Josh Schwerin, a Clinton campaign spokesman, issued a statement Thursday saying that "it's disappointing Senator Sanders and his campaign strategists have chosen to change direction and engage in the type of personal attacks that they previously said he wouldn't do."

Sanders’s camp countered by pointing out that he had made similar statements about the investigation into Clinton’s emails before and immediately after the Las Vegas debate -- suggesting there was nothing new in his comments to The Journal.

Among the examples cited were these comments from Sanders in an interview on CNN after the debate: "There is a process in place for the email situation that Hillary Clinton is dealing with.  Let it play itself out."