Maverick Republican Sen. John McCain of Arizona, in a widening rift with President Bush and his party's dominant conservative wing, is talking with advisers about leaving the GOP and launching a third-party challenge to Bush in 2004, those close to the senator say.

Such a move is not imminent, they say. For the near term, McCain, who upset Bush in the 2000 New Hampshire primary and won 5 million primary-season votes, will work to build a centrist faction within the GOP to mirror the moderate "New Democrats."

But if Bush struggles as president, and if McCain loses on key issues such as defense funding and campaign finance reform, advisers say he may challenge Bush in the same way the reformist Teddy Roosevelt, McCain's hero, battled a conservative Republican, William Howard Taft, in 1912.

This weekend, McCain is hosting the Senate Democratic leader, Thomas A. Daschle (S.D.), at his home in Sedona, Ariz. His office called it a "social event," but McCain has met privately to discuss party switching with at least three Democratic senators -- Daschle, Edward M. Kennedy (Mass.) and John Edwards (N.C.).

On Thursday, four McCain loyalists -- campaign strategist John Weaver, legislative director Daniel McKivergan, Weekly Standard magazine publisher William Kristol and Hudson Institute scholar Marshall Wittmann -- met over lunch to debate whether McCain should quit the GOP.

McCain met in recent weeks with Will Marshall, a top official of the Democratic Leadership Council, to discuss similarities over national service, tax and environmental policies. "I was struck by how much we were in common," Marshall said. "It's an intriguing development."

Over the last two years, McCain has undergone a virtual ideological conversion, severing almost all ties to the right wing of the GOP. In addition to supporting legislation adamantly opposed by most of his Republican colleagues, he has joined Democrats in becoming a leading sponsor of patients' rights, fewer tax breaks for the rich and new gun control measures.

Asked if he plans to run for president again as a Republican or independent, McCain said, "I don't envision running again." He said Democrats "approached me a couple of times" to discuss party switching, but he said he told them he has "no cause to leave the Republican Party, period." He said his willingness to talk should not be interpreted as a signal of his willingness to abandon the GOP. McCain aides say his only goal for the moment is to influence the Republican Party.

But McCain's confidants and advisers -- some of the same people who persuaded him to launch his presidential candidacy for 2000 -- are pushing him in that direction.

"I would say it's fifty-fifty right now -- it's an open question right down the middle," said Wittmann. If Bush were to veto campaign finance reform or the House were to kill it, "that could be a precipitating event," he said.

Kristol added: "I think McCain honestly doesn't know what he's going to do but is open to all possibilities."

Whether or not McCain leaves the GOP, he has transformed himself from quirky conservative before the 2000 campaign to spokesman for an embattled progressive wing of the Republican Party today. Whatever McCain does, it is clear he will continue to be a thorn in the side of Bush, who is already weakened by the defection from the GOP of moderate Sen. James M. Jeffords of Vermont.

"John McCain was radicalized by the [2000] primary process in two ways," said former party chairman Richard N. Bond. "First, he was totally alienated by the conservative wing of the Republican Party. Second, with the influx of independents who came into his campaign, he moved from center-right to center-left."

McCain was deeply angered during the South Carolina primary because he thought Bush authorized an attack on his Vietnam War credentials and a sustained assault on his commitment to conservative social values by the locally powerful Christian right.

Pressure on McCain to bolt his party intensified yesterday with the report in Human Events, a conservative weekly, of plans by Arizona Republicans to run a challenger to McCain in the 2004 Senate primary.

McCain's outspoken support of campaign finance reform and other legislation favored by Democrats has turned him into a pariah among conservatives. On FreeRepublic.com's "Conservative News Forum," McCain is referred to as "Komrade McCain," "McLame" and "McBigHead." Contributors to the Web site regularly challenge the patriotism of the former prisoner of war: "The man, far from being the war hero he pompously claims to be, was actually a traitor."

Further exacerbating tensions, his key backers are charging with growing vehemence that the Bush White House and GOP Senate and House leaders are conducting a vendetta against McCain loyalists -- cutting them off from government and political jobs, undermining their status with current employers and excluding McCain from key Republican parlays.

The senator's allies also charge that a person who gave the McCain campaign $250 said he had been up for a post in the State Department, but, when he was interviewed by White House staff, he was told a "red flag" had come up: the $250 donation.

They also say that in New Hampshire, pro-Bush forces conducted a failed bid to recall the state's two Republican National Committee members, who had backed McCain. Also, Weaver has told associates that Karl Rove, a top aide to Bush, intervened and prevented Weaver from being hired as a consultant to the reelection campaign of Jeff Sessions (R-Ala.). Rove declined to comment, and the chairman of the Sessions campaign said he was never pressured by Rove.

In an interview, McCain said, "I don't mind any criticism of me, but I don't think someone in their twenties should not be allowed to pursue a career in public service" because of past political commitments.

White House officials adamantly deny the charges and offered a detailed rebuttal of the complaints. They provided a list of McCain donors and supporters who have gotten high-level administration jobs, and they contended that a contribution to McCain is not treated as a "red flag." Some of McCain's top advisers, White House officials said, are perpetually portraying themselves as "victims" of power wielded unfairly by Bush.

"The president and the senator have a good relationship, and I see nothing that will change that," said Ari Fleischer, Bush's press secretary. Asked about discrimination against McCain loyalists, Fleischer pointed to Torie Clark, a former McCain aide who is now Pentagon spokeswoman.

"Senator McCain also has been invited to the White House on seven occasions since the administration took office," Fleischer added.

McCain has been rebuffed by the GOP House and Senate leadership on a number of fronts, and he faces major hurdles trying to become a force within the GOP.

A veteran Republican strategist remarked: "I don't know what he is trying to do, but if you were trying to offend everybody, this is the way to go about it in the things he says, in the things his staff and political operatives say, and of course in becoming the favorite co-sponsor with every liberal Democrat of legislation that the vast majority of Republicans oppose."

Grover Norquist, a GOP strategist and a Bush ally, said: "He is not drifting right. He is not drifting left. He is drifting in front of the television cameras. He will do whatever gets him the most attention, and two weeks from now I don't have a clue where he will be."

While the Republican right is infuriated with McCain, the Democratic center-left is intrigued.

"He seems to be changing rapidly before our eyes," said Robert McIntyre, head of Citizens for Tax Justice and a major critic of the Bush tax cut. "It's very interesting. It's almost like he stopped hanging around with the wrong people, and has woken up."

McCain's advisers position their man as a progressive and a populist in the Teddy Roosevelt image, with a dash of John F. Kennedy's national service and Ronald Reagan's folksiness.

The superficial similarities with Roosevelt are obvious: Roosevelt, too, was a larger-than-life personality and a war hero who had a reputation as the public's protector against corporate greed. His Republican rival, like Bush today, appeared sympathetic to business interests. There were charges that the Taft White House was mistreating Roosevelt appointees and family members. Roosevelt campaigned on disclosure of campaign expenditures, regulating interstate industry, a minimum wage, unemployment insurance and old-age pensions.

The Roosevelt-McCain parallel is disputed by some.

"The fundamental difference is he [Roosevelt] went to war [with the party] after he was president," said Steven Rosenstone, a University of Minnesota expert on third parties. The Roosevelt-McCain comparison "sounds like a stretch to me."

McCain's agenda, and that of a prospective McCain-led third party, is a hawkish foreign policy, domestic reform and a call for universal national service for young Americans. McCain sees each party held hostage by its base -- Democrats wedded to entitlements and Republicans dominated by corporate interests -- thus leaving room for a centrist populism.

A couple of McCain's advisers have convinced themselves he could win the presidency in 2004 as a third-party candidate. Others suggest that even if he lost, he could reshape politics more to his liking for years to come.

"You have deadlock between the parties, and the ideological forces that drove the two parties are somewhat spent and exhausted," Kristol said.

The conservative era that began with Richard Nixon is waning. "It feels like we're at the end of this period in American politics," Kristol said.

Sen. John McCain has met with Democratic senators.