When first lady Hillary Rodham Clinton flies to Upstate New York Wednesday to formally begin her Senate exploratory campaign, she'll be riding on the first big controversy of the nascent campaign: a gleaming blue-and-white Air Force passenger jet.

A candidacy without precedent is raising murky ethical questions. The first lady's military travel -- which White House officials say is a security requirement and Republicans are protesting as an illegitimate government subsidy -- is only the most prominent example.

Aides to the first lady say they have been studying long-established rules about what political activities are allowed by White House employees. Soon, they say, her staff will have separate phones, fax lines and computers at the White House to be used exclusively for political work, so as not to run afoul of Hatch Act prohibitions on using federal resources for campaign purposes. The same thing was done during President Clinton's reelection campaign in 1996.

"It's unprecedented that we have a first lady [exploring a candidacy], but it is not unprecedented that we have these rules to be followed," said Melanne Verveer, the first lady's chief of staff. "The framework is there . . . and we have to apply it appropriately."

Once her exploratory campaign is underway, the first lady and some of her entourage will reimburse the government for political travel at a rate equal to a first-class fare. But GOP critics and some public interest groups note correctly that this is only a fraction of the costs incurred.

Many of the questions her candidacy raises cannot be resolved by installing office equipment or writing a check for a plane flight.

The first lady, for instance, joined a White House meeting Thursday with a delegation of New Yorkers upset about Medicare reimbursements for Empire State teaching hospitals. Was she there advocating the interests of taxpayers of all 50 states or the interests of New Yorkers who believe they are entitled to a large slice of the Medicare pie?

Aides to Hillary Clinton said yesterday that she was there representing concerns about reimbursement held by teaching hospitals nationally, though they acknowledged she attended the meeting at the urging of Sen. Charles E. Schumer (D-N.Y.). They said she is determined to continue the responsibilities she has taken upon herself as first lady, even as she plans to spend between one-third and half her time over the next couple of months testing the New York waters.

And they vow Clinton will avoid putting the vast resources at her disposal in the role of first lady in the service of her role as likely candidate. But she will begin her explorations next week laboring under criticism that she has already broken this pledge, with the 15 days she has been in New York state this year (12 on her own, three at least in part with her husband).

Republican National Committee Chairman Jim Nicholson said yesterday that these trips were part of a de facto exploratory campaign for which taxpayers were unfairly charged. "It does not look right, it does not smell right; she should reimburse," he said. "She is up there vetting New York and she has been all year, and they don't deny that."

The first lady's office does deny this. Aides said next week will mark her first trip to New York that will be "exploratory" in nature. The majority of her time in New York this year, they said, has been official. And they note that the part of her travel that has been political -- mostly Democratic fund-raisers -- has been reimbursed by the groups that hosted her.

But White House officials declined to say what the amount of reimbursement has been (and said they don't know the total themselves.) The billing procedures for reimbursing official travel -- in which different military and administrative offices send out bills for different parts of the same trip -- make it hard to calculate this figure, according to the officials.

These officials yesterday disclosed for the first time the proportion of the first lady's 1999 New York travel that was deemed by White House lawyers to be political versus official. Cumulatively, for the 12 days she traveled without the president, the White House figures show that just shy of 70 percent of her time was spent ostensibly on official business.

Different days had different proportions. On April 22, Hillary Clinton addressed an award ceremony for a teachers union in Niagara Falls, an event deemed 100 percent official. A trip June 9-10 took her to a public school in New York City (official) and a fund-raiser in Binghamton for Rep. Maurice D. Hinchey (political). The breakdown of the trip was 55 percent official, 45 percent political. The Hinchey campaign was billed for 45 percent of the travel costs of the trip.

Under Federal Election Commission rules, the first lady's appearances on behalf of other candidates must be reimbursed at coach travel rates for herself and some staff. Once she becomes an exploratory candidate, she must reimburse at first-class rates. And she is no longer allowed to allocate costs between official business and political. If even a part of a trip is political, she must reimburse for the whole thing; traveling staff will still be able to allocate their costs.

No one at the White House asserts that these reimbursement rates even begin to pay for the actual costs. While two or three political aides who travel with her must reimburse, other personnel -- including Secret Service agents and Air Force staff -- who accompany her are always deemed official. And even first-class reimbursement is a pittance compared with the actual cost of flying either of the two jets she usually uses. A C-9 jet, similar to a civilian DC-9, costs $2,141 per hour to fly, said Sgt. Rick Corral, a spokesman for Andrews Air Force Base. A smaller C-20, an executive jet, costs even more: $4,059 per hour.

Nicholson and other Republicans flew around New York (on a chartered plane that cost $10,000 for the day) Thursday to highlight a GOP study estimating that the first lady has spent $220,000 in travel in 1999. Among those who helped prepare the study was Billy R. Dale, the former White House travel office director fired in a shake-up encouraged by the first lady in 1993.

Nicholson said there is no reason that the first lady, unlike the president or vice president, cannot fly on a plane she charters, big enough for the Secret Service to fly along: "She can be adequately secured on [charter] planes." He added that it is a "tremendous advantage" for her to fly on the military planes, allowing her to avoid the cost of planes that other New York candidates routinely use.

Unlike the president, the first lady is able to decline Secret Service protection or use of Air Force planes if she wants to, said Secret Service spokesman Jim Mackin. But he added: "That would be against our strongest recommendations."

Charles Lewis, head of the Center for Public Integrity, said that if the first lady travels on military jets, she should reimburse for the entire cost. He urged a thorough accounting of how costs have been reimbursed to date: "That information exists and it should be made available."

CAPTION: First lady Hillary Rodham Clinton follows Rep. Maurice D. Hinchey off plane at Binghamton, N.Y., airport last month, when she appeared at a fund-raiser for Hinchey and at a public school in New York City. Because her trip was regarded as part political and part official, the Hinchey campaign was billed for 45 percent of her travel expenses.