NEW YORK (CBSNewYork/AP) — Staten Island Rep. Michael Grimm has pleaded guilty to a federal tax evasion charge that he fought as he won re-election this fall.

Grimm entered the plea Tuesday in federal court in Brooklyn in a case stemming from an investigation of his campaign financing.

He pleaded guilty to one count of aiding in the filing of a false tax return. He’d been set to go to trial in February.

As CBS2’s Dave Carlin reported, the congressman’s public confession was a stunning turn around for a man who had spent years maintaining his innocence.

“I should not have done it and I’m truly sorry for it,” Grimm said.

Rep. Michael Grimm Pleads Guilty To Federal Tax Evasion Charge

Sentencing is set for June 8. Prosecutors said between 24 and 30 months in prison would be appropriate; the defense estimated the appropriate sentence as 12 to 18 months.

According to an indictment, the tax fraud began in 2007 after Grimm retired from the FBI and began investing in a small Manhattan restaurant called Healthalicious.

The indictment accused him of underreporting more than $1 million in wages and receipts to evade payroll, income and sales taxes, partly by paying immigrant workers, some of them in the country illegally, in cash.

“Although it was a little restaurant, I made some very big mistakes; mistakes that I immensely, immensely regret,” Grimm said.

The case stemmed from an investigation of Grimm’s campaign financing. He was never charged with any offense related to his campaign, but a woman romantically linked to him pleaded guilty in September to lining up straw donors for his 2010 run.

The plea brings an end to the four-year investigation against him, but could also bring with it a prison sentence and possibly Grimm’s resignation.