New York Republican Rep. Michael Grimm pleaded guilty on Tuesday to a federal tax evasion charge, throwing his future in Congress into question just weeks after he won re-election.

Grimm entered the guilty plea, to one count of tax evasion, in federal court in Brooklyn. Under the plea agreement, he could face up to three years in federal prison -- his sentencing is set for June 8.

The State Island congressman, leaving the court, told reporters he's sorry for mistakes he made as a small business owner but stressed that all this happened before he entered Congress. Asked if he would resign, he said he'd serve as long as he is able.

But even before he formally entered the plea, Democratic officials were calling on him to resign.

"Now that the election is over, Congressman Grimm is finally admitting the truth to his constituents," House Democratic Leader Nancy Pelosi said. "Clearly, Speaker Boehner must insist that Congressman Grimm resign immediately."

Speaker John Boehner's spokesman, Michael Steel, said: "We won't have any announcements until the speaker discusses the matter with Mr. Grimm."

A senior GOP aide also said Pelosi has "zero credibility" on the issue after standing behind other Democrats accused of wrongdoing.

The indictment against Grimm alleged the tax fraud began in 2007 after he retired from the FBI and began investing in a Manhattan eatery called Healthalicious. It accused him of underreporting more than $1 million in wages and receipts to evade payroll, income and sales taxes, in part by paying immigrant workers, some of them in the country illegally, in cash.

In court on Tuesday, Grimm acknowledged that while operating the restaurant, "we underestimated gross receipts and used money to pay employees off the books. The tax returns were false."

Judge Pamela Chin made no modification to his bail -- he's free on $400,000 bond -- and Grimm was able to leave the courthouse.

The congressman won re-election in November while fighting the charges. His comments on whether he'd resign echoed comments he made during an October debate.

Asked whether he would resign if found guilty, he responded, "Certainly, if I was not able to serve, then of course I would step aside and there would be a special election."

Grimm, 44, made headlines in January after telling a local cable TV news station reporter he wanted to throw him off a balcony in the Capitol for asking about the campaign finance inquiry.

The lawmaker was never charged with fundraising fraud. But a Texas woman romantically linked to him, Diana Durand, pleaded guilty in September to charges she lined up straw donors for his 2010 campaign.

Durand admitted she had asked "two friends" to make $12,000 contributions to Grimm and another politician and then secretly reimbursed them as a way to circumvent campaign contribution limits. She denied through her attorney that Grimm knew anything about it.

Fox News' Melanie Schuman and Mike Emanuel and The Associated Press contributed to this report.