SAN DIEGO — Rep. Duncan D. Hunter pleaded guilty Tuesday to one count of misusing campaign funds after denying for more than a year that he and his wife had tapped a quarter of a million dollars in campaign money to pay for personal expenses.

Hunter (R-Calif.) entered the plea on conspiring to misuse the funds — one of the four charges — in the U.S. District Court for the Southern District of California. He will be sentenced March 17 and faces up to five years in prison and a $250,000 fine.

“I failed to monitor and account for my campaign spending. I made mistakes,” he said. He declined to answer reporters’ questions.

The guilty plea is an ignominious event for the six-term congressman and Marine veteran who was elected in 2008 to a seat his father, Duncan L. Hunter, held for 28 years. It was not immediately clear whether the younger Hunter would resign his seat.

The proceedings took less than 10 minutes as Hunter, flanked by his attorney, faced the judge. His father sat behind him.

In a statement outside the courthouse, Phil Halpern, who prosecuted the case for the federal government, acknowledged the congressman’s military and public service career and highlighted his three tours of duty “in three war zones.” But Halpern also said Hunter is not above the law.

“The government takes little pleasure in today’s announcement. Yet it is the very foundation of our system of law that no one, no one is above it,” Halpern said. “No figure, regardless of the heights they’ve reached, no figure, regardless of what they’ve contributed, and no figure, regardless of what office they occupy, should be allowed in this country to cry witch hunt or fake news — an attempt to deflect their criminal wrongdoings.”

Halpern said the government would seek a prison term for Hunter.

Hunter and his wife, Margaret Hunter, were charged in 2018 with using more than $250,000 in campaign funds to pay for personal expenses, including family vacations, theater tickets and school tuition.

Duncan D. Hunter has continued serving in Congress and won reelection last year despite the charges, although he was stripped of his committee assignments.

“As most folks know, my trial was set for January 22nd,” Hunter, who previously pleaded not guilty, said in an interview Monday with San ­Diego-based KUSI-TV. “That’s not going to happen now. Tomorrow, on Tuesday, I’m going to change my plea to guilty. I think it’s important not to have a public trial for three reasons. And those three reasons are my kids.”

The congressman said only that he believes it is important to keep the seat in GOP hands because President Trump “right now needs more support than ever.”

“I’m confident that the transition will be a good one,” Hunter said. “My office is going to remain open. I’ve got a great staff. We’re going to handle people’s cases, and we’re going to pass it off to whoever takes this seat next.”

Also this year, Rep. Chris Collins (R-N.Y.) resigned from Congress after pleading guilty to insider trading charges.

Hunter faced increasingly tough political odds, even from within the GOP: Former congressman Darrell Issa (R-Calif.) and former San Diego city councilman Carl DeMaio recently launched primary challenges against Hunter. And Democrat Ammar Campa-Najjar, who lost narrowly to Hunter last year, is pursuing another bid for the seat.

Hunter’s legal situation has also become more precarious in recent months. In June, his wife pleaded guilty to one count of conspiring with her husband to spend $25,000 in campaign funds for personal use. As part of the deal, she agreed to cooperate with prosecutors and testify against her husband.

Later that month, federal prosecutors alleged that the congressman had also used campaign money to fund trips, dinners and drinks with women with whom he was romantically involved — three lobbyists, a woman who worked in his congressional office and another who worked for a member of the House leadership.

Hunter told KUSI on Monday that he wants the public to know that he “did make mistakes” but that “not a single dime of taxpayer money is involved in this.”

Felicia Sonmez contributed to this report.