Image copyright Getty Images Image caption Hunter outside the San Diego courthouse in August 2018 (FILE)

US Congressman Duncan Hunter has pleaded guilty to misusing campaign funds after months of decrying the probe as a political "witchhunt".

The six-term Republican from California was accused by prosecutors in June of using the fund to finance family expenses and extramarital affairs.

He initially pleaded innocent, but formally changed his plea in a federal courthouse on Tuesday.

Prosecutors say he plans to resign his congressional seat.

According to prosecutors, Hunter and his wife Margaret Hunter - the campaign treasurer - raided the campaign war chest to spend more than $250,000 (£190,000) on themselves.

Expenses included a family trip to Italy, video games, golf outings, groceries, bedding and a $600 airline ticket for their pet rabbit, Eggbert.

Using campaign funds for personal use is illegal because it could give undue influence to financial contributors.

Hunter, 42, originally claimed the expenses were made in error by his wife and son and reimbursed his campaign at least $62,000.

"She was also the campaign manager, so whatever she did that'll be looked at too, I'm sure," he told Fox News in August 2018, in reference to his wife.

He also claimed to voters that prosecutors were biased against him, saying that they had attended a fundraiser for Democratic 2016 presidential candidate Hillary Clinton, and therefore would not give him a fair trial.

In an interview on Monday before changing his plea, Hunter said he was making the decision in order to spare his wife from going to jail.

"I think my kids need a mom in the home," he told KUSI News, adding: "Whatever my time in custody will be, I will take that hit."

"I think it's important that people know that I did make mistakes," he continued. "I did not properly monitor or account for my campaign money. I justify my plea with the understanding that I am responsible for my own campaign and my own campaign money."

Last June, Margaret Hunter accepted a plea deal that could have seen her testify against her husband. Both Duncan and Margaret Hunter are each facing five years in prison.

In court, prosecutor Phil Halpern criticised Hunter for claiming that the investigation was politically motivated.

"No figure, regardless of what office they occupy, should be allowed in this country to cry witchhunt or fake news and attempt to deflect their criminal wrongdoings," he said.

Who is Duncan Hunter?

The first US Marine veteran of both Iraq and Afghanistan elected to congress succeeded his father in 2008, who had represented the San Diego area since 1980.

In 2016, he was one of the first sitting congressman to support US President Donald Trump's campaign. Another early endorser, ex-New York Congressman Chris Collins pleaded guilty to insider trading in September after initially maintaining his innocence and winning another term.

Last year, Hunter was re-elected, despite being under federal indictment.

He is due to be sentenced on 17 March.