Arizona Sen. John McCain called on his presidential rivals yesterday "to lead by example" in combating "corrosive cynicism" about the big money-dominated political system even as he defended his own intervention with a federal agency on behalf of a major campaign benefactor.

In a day of awkward juxtapositions for a candidate who has made the drive to clean up Washington's "swamp" of special-interest politics the centerpiece of his campaign, McCain called "legitimate" and "appropriate" a letter he wrote last month to the Federal Communications Commission. McCain urged a quick vote on a long-delayed deal involving a family broadcasting network, Paxson Communications--in a letter written at the behest of the firm's Washington lobbyist.

McCain has flown aboard Paxson's corporate jet four times in recent months and has received more than $20,000 in contributions from company executives for his presidential bid. On Dec. 9, the company's jet ferried him to a Florida fund-raiser that was held aboard a yacht in West Palm Beach. The next day, McCain sent his letter to the FCC in his capacity as chairman of the Senate Commerce Committee, urging a speedy vote on Paxson's proposed purchase of a Pittsburgh television station.

A week later, the FCC approved the deal. But at the same time, the commission's chairman, William E. Kennard, chided McCain in a letter for his "highly unusual" request. Angela J. Campbell, a lawyer for opponents of the sale, charged in an interview that McCain's letter was a "heavy-handed" and improper use of his influence.

As chairman of the influential commerce panel, McCain has often feuded with the agency that oversees telecommunications policy. In response to a Boston Globe report about his Paxson ties, McCain yesterday called the FCC "the least efficient, most bureaucratic, least responsive bureaucracy" in the capital.

His letter, McCain said, carefully steered clear of urging approval of the Paxson deal, and his campaign later said it had found about 20 similar McCain requests from the past few years demanding faster action by the FCC on various regulatory matters. Campaign spokesman Dan Schnur said that several of those letters were also on behalf of campaign contributors, including AT&T, Tribune Broadcasting and Motorola.

As for the Paxson letter, McCain's aides confirmed that he had written the missive at the request of Alcalde & Fay, the Washington lobbying firm retained by Paxson. Last night, Schnur said that Paxson's chairman, Lowell W. "Bud" Paxson, was scheduled to host a fund-raiser for McCain this weekend. Soon after, McCain told reporters he was canceling the fund-raiser because it would be inappropriate.

In many ways, the episode offers a classic example of how Washington works, intertwining campaign contributions, corporate lobbyists and a powerful committee chairman. For McCain, it highlights the perils of running a campaign that stresses the evils of a system in which he aggressively participates.

Throughout the presidential contest, McCain has encountered questions about whether corporate interests are bankrolling his campaign to curry favor with a powerful chairman. Lobbyists for large telecommunications and transportation companies have helped spearhead his Washington fund-raising effort, while such firms as BellSouth Corp. and Union Pacific Corp. have made their aircraft available for his campaign travel.

And last spring, McCain had another pointed exchange with the FCC's Kennard, the Globe reported, accusing the FCC of stalling the planned merger between two regional phone companies, Ameritech and SBC Communications. Ameritech's chairman, Richard Notabaert, has been a top McCain backer, hosting a fund-raiser for him in Chicago last May. SBC's lobbyist, Timothy McKone, helped arrange a March dinner for McCain at a District steakhouse.

Yesterday morning, McCain told reporters that the Paxson case "highlights the need for legitimate campaign finance reform" to remove the appearance of impropriety from the system.

"The system corrupts all of us," he said aboard his campaign bus "the Straight Talk Express."

By midday, McCain had returned to the moral high ground, delivering what aides billed as a major speech to the Boys and Girls Club in Manchester, N.H., in which he stressed the themes of reform that have helped fuel his rise in the polls in the state's first-in-the-nation Feb. 1 primary.

McCain asserted that "pervasive and growing cynicism" toward government and politics casts a dark shadow over the country's future, while urging his rivals in both parties to refrain from attack politics, negative ads and campaign spin.

"I am aware that as a candidate for president, I must accept a special obligation to conduct my campaign in a way that will not just avoid exacerbating public cynicism about politics, but help, I hope, to alleviate it," he said.

McCain's chief rival for the GOP nomination, Texas Gov. George W. Bush, said in Iowa today: "I think it's really important for people who advocate reforms to live to the spirit of the reforms they advocate."

Among those most vocally defending McCain yesterday back in Washington were several of the "big-time K Street lobbyists" he has been inveighing against on the campaign trail.

McCain "has done what many, many, many others have done," said Hector Alcalde, the lobbying veteran who is Paxson's chief D.C. representative. "Everybody knows that it is not highly unusual for a member of Congress with oversight over an agency to write urging them to act."

Lanny J. Davis, a lobbyist with Patton Boggs who was the Clinton White House's special counsel during the campaign finance scandal, agreed with the this-is-how-Washington-works defense. "All McCain is doing is saying you have no right to refuse to act," Davis said. "If the member of Congress with jurisdiction over the FCC doesn't have an obligation to write that letter, that means the FCC has no oversight at all."

Davis's role in the lobbying battle waged over the Pittsburgh station deal was to enlist support from Democratic members of Congress, and he said his firm was responsible for three letters to the FCC--from Reps. Steny H. Hoyer (D-Md.), Tom Udall (D-N.M.) and Ron Klink (D-Pa.)--supporting the complicated sale of WQED's broadcast license.

Several other lawmakers also wrote the FCC to endorse the plan, and both lobbyists professed to be disappointed that McCain's missive had stopped short of advocating the sale. "He refused to do what Paxson wanted him to do and take a position on the merits, unlike a number of Democrats," Davis said. "If anything, this exemplifies Senator McCain's refusal to do the bidding of a contributor."

Other lobbyists, however, said they believed that McCain may have crossed a line by appearing to urge action by Dec. 15 and pointedly asking the commissioners to explain their action or lack thereof. "Writing a letter to the FCC expressing concern is perfectly common and ordinary," said a lobbyist who is a former agency staffer. "But to tell the commissioners when to vote on something is not. It's very unusual."

Glasser reported from Washington, Balz from New Hampshire. Staff writers Terry M. Neal in Iowa and Ben White in Washington contributed to this report.

CAPTION: Sen. John McCain (R-Ariz.), who is seeking the GOP presidential nomination, talks to Boys and Girls Club of Manchester, N.H.