WASHINGTON (Reuters) - Federal prosecutors intend to bring criminal charges against Michael Grimm, a Republican U.S. congressman from New York who has been under investigation for campaign finance violations, his lawyer said on Friday.

Attorney William McGinley condemned the expected charges as a "politically driven vendetta" against the congressman, who generated headlines in January when he threatened to throw a television reporter off a balcony after an interview in the U.S. Capitol building.

Another source familiar with the case, speaking on condition of anonymity, confirmed that charges against Grimm were likely.

Grimm, a former Marine, was elected to the House of Representatives in 2010 as one of a wave of conservatives backed by the Tea Party movement, which advocates small government and minimal taxes. Federal investigators have been looking into his political fundraising for at least two years.

Grimm's confrontation with the reporter, which was caught on camera, took place on the evening of President Barack Obama's State of the Union address to Congress.

"I will break you in half," Grimm told the reporter with cable channel NY1 News, who had asked the Republican about the federal investigation into possible campaign finance violations.

McGinley, his lawyer, said the congressman ultimately will be vindicated of the charges.

"After more than two years of investigation plagued by malicious leaks, violations of grand jury secrecy, and strong-arm tactics, the U.S. Attorney's Office has disclosed its intent to file criminal charges against Congressman Grimm," McGinley said in a statement.

"We are disappointed by the government's decision, but hardly surprised. From the beginning, the government has pursued a politically driven vendetta against Congressman Grimm and not an independent search for the truth. Congressman Grimm asserts his innocence of any wrongdoing," the lawyer added.

Diana Durand, a fundraiser for Grimm, was arrested in January on charges that she illegally funneled more than $10,000 to his campaign.

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The House ethics committee opened a formal probe into the congressman's campaign finance activities in 2012, but unanimously voted to defer the inquiry in light of the Justice Department's investigation.

The committee's probe had focused on allegations that Grimm broke federal campaign finance laws by soliciting and accepting prohibited campaign contributions, filed false campaign finance reports and improperly sought assistance from a foreign national in soliciting campaign contributions in exchange for helping get a "green card" signifying permanent U.S. residence.

Grimm, who represents New York City's borough of Staten Island, grew up in New York, dropped out of college to join the Marines and served in the Gulf War, then joined the FBI as a clerk while going to college at night.

He eventually became a special agent investigating financial fraud and organized crime before leaving the FBI to open a health food store.

(Reporting by Julia Edwards in Washington and Emily Flitter in New York; Writing by Will Dunham; Editing by David Storey)