LOS ANGELES — Just a few months after taking his seat in Congress in 2009, Duncan Hunter began an affair with a lobbyist and used campaign money to pay for dates, prosecutors said in a new court filing.

[Update: Duncan Hunter will plead guilty to misuse of campaign funds.]

On one of their first “solo getaways,” according to prosecutors, the pair went on a ski trip to the Heavenly resort near Lake Tahoe and “spent the weekend skiing, ordering room service and enjoying the amenities of the full service resort.” The hotel tab: $1,008. And no expense seemed too small that it couldn’t be paid for with campaign money, not even the $7 Sam Adams he drank at the hotel bar just after checking in, according to the filing.

Mr. Hunter, a Republican, has been accused of spending almost $250,000 of campaign funds on personal expenses. The sprawling indictment issued last year threatened the re-election campaign of Mr. Hunter, an Iraq war veteran, but he won in his conservative district in San Diego County, where many military veterans live. Mr. Hunter has pleaded not guilty and is awaiting a trial scheduled to begin in September. His wife, who was also indicted, recently pleaded guilty and agreed to cooperate with prosecutors.

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The 47-page indictment last year of Mr. Hunter, a scion of a political dynasty in Southern California where his father was a congressman for almost three decades, accused him of a brazen pattern of financial misconduct, including using campaign money for an elaborate vacation in Italy. Prosecutors also said Mr. Hunter had tried to pass off personal expenses as charitable contributions for veterans, an offense that alienated an important constituency.