Sen. John McCain's wife flew in her company's private jet last year from Phoenix to New York City, where she spent three hours of the two-day trip at a fund-raiser for her husband's presidential campaign. The rest of her visit was devoted to personal matters, according to the campaign.

The campaign didn't reimburse Cindy McCain for the March 7-9, 2007, trip, which included the event near Times Square that brought in an estimated $100,000. Should it have done so?

According to federal campaign finance rules, the answer is yes -- and no, depending on which portion of the code is applied.

Mrs. McCain's trip offers a good example of the complex and sometimes contradictory ways that post-Watergate regulations have evolved to dictate what is considered proper use of campaign funds. Sen. McCain is one of the leading champions of such rules, but critics charge his campaign has applied them liberally in a number of instances.

In 1977, the Federal Election Commission sought to prevent politicians and others campaigning for them from getting free travel to political events on donors' corporate planes or government flights. Thus, paragraph 106.3(b)(3) of Title 11 of the Code of Federal Regulations was born, requiring campaigns to foot the bill for an entire leg of a trip if it includes anything more than "incidental" campaign work. Classifying and reporting these trips requires lawyers to examine all a candidate's activities and sometimes measure hypothetical legs of a multistop tour to keep payments separate.

Then in 1995, the commission felt compelled to respond to a scandal over candidates taking vacations on the campaign's bill or otherwise using campaign money as a personal bank account simply by including a token campaign element in their expenses. The FEC created a "personal use" prohibition that required candidates and others working on their behalf to pay out of their own pockets certain expenses that would have existed irrespective of the campaign -- and blocking the candidates from being reimbursed by the campaign for such expenses. This one is paragraph 113.1(g) of Title 11 in the same code.

In 2002, the commission recognized the contradiction between 106.3(b)(3) and 113.1(g) in an advisory opinion requested by the city of Bettendorf, Iowa. The mayor made an official visit to Washington with other dignitaries, but stayed longer to attend a candidate training event. As the training was on personal time and the trip would have happened anyway, the commission ruled that her campaign didn't need to reimburse the city for the airfare.

This ruling is interpreted differently by different candidates, and its significance is in doubt. Sen. McCain's campaign says it gives Mrs. McCain the right to use her company's jet without the campaign picking up the tab. The plane is owned by King Aviation LLC, a subsidiary of her family's beer distributorship, Hensley & Co., where Mrs. McCain is chairman.

She used the jet on several trips last year that included campaign-related activity but never got campaign reimbursement, according to flight-tracking records and campaign-finance reports verified by the McCain campaign. At the New York fund-raiser, she spoke on stage, warming up the audience for her husband.

If the campaign had paid for Mrs. McCain's trip to New York and three others that appear to have included some campaign work, it would likely have cost a total of about $15,000, the equivalent of first-class fare for the trips combined.

Jan Baran, a Republican campaign lawyer, said the campaign should have paid. "I don't know why they want to fight it," he said. "The chutzpah is not that they're not paying for this trip, it's that they're using a corporate airplane at a highly discounted rate."

Cleta Mitchell, another Republican lawyer, said she would have advised her clients to pay for such a trip, but that the McCain campaign had a solid legal basis for not doing so. "That advisory opinion is something of an outlier," she said, "but I think it is probably appropriate that they are relying on that to say that, under the circumstances, it would have been inappropriate to pay for her air travel."

The FEC provides a mechanism to resolve such questions. Campaigns can submit the facts of their quandary to the commission and request an opinion from the body. But that's been impossible for the past six months. The six-member panel currently has only two members, short of the four needed to pass rulings. An impasse between the White House and Congress over the nominees has blocked a vote on replacements. A vote on five new commissioners may come as soon as this week.