When Sen. John McCain (R-Ariz.) takes the Senate floor this week to begin another uphill fight for tougher campaign finance laws, he will find himself in a familiar position: enchanting reform advocates, infuriating Republican colleagues--and probably losing.

In his 17 years on Capitol Hill, McCain--now a candidate for the GOP presidential nomination--has become one of Congress's leading cage-rattlers, a conservative with a populist and reformist bent who is better known for the fights he has lost than those he has won.

While most of the focus of his latest campaign has been on his compelling personal history, including heroism and captivity during the Vietnam War, McCain's record in Congress also offers important clues to how he would perform in the White House.

He has won some fights, such as banning lobbyists' gifts to lawmakers and increasing the amount that Social Security recipients can earn without losing benefits. He has lost more, including legislation to strengthen campaign fund-raising regulation and to implement the national tobacco settlement. He has been an influential internationalist in debates over military and foreign policy, a daring but less persuasive force on domestic matters and a bipartisan player in both arenas.

At one time or another, the soldier-turned-lawmaker, 63, has found himself at odds with the tobacco industry, the Pentagon, defense contractors, government regulators, K Street lobbyists, campaign fund-raisers, broadcasters, tax-dodging corporations, isolationists within the GOP, conservative advocacy groups and, time after time, his own party leaders on specific issues.

"I've had a lot of successes and some very significant failures. . . . This is what happens sometimes when you take on very, very big issues," McCain said in summing up his record in a recent interview.

Few of his colleagues are neutral on the subject of the personally engaging but prickly McCain. Friends regard him as a gutsy, principled crusader. Foes see him as an ambitious, flashy grandstander. Renowned for his short temper and habit of going nose-to-nose with colleagues on the Senate floor, McCain acknowledges he would not win any congeniality awards. Some say this underscores his strong-willed independence. Others say it could be scary in a president.

"He's one of the few people left who really gets upset about issues and he's not at all quiet about it," said Sen. Fred D. Thompson (R-Tenn.), a friend and ally. "John can be impatient sometimes and that rubs people the wrong way."

Among his admirers is Rep. James P. Moran (D-Va.), who has tangled sometimes acrimoniously with McCain over the senator's campaign for more landings and takeoffs at Reagan National Airport. Much as they disagree, Moran says he believes McCain acts out of a firm belief in marketplace competition. "He's got integrity, he's got backbone, he's got guts, you know where he stands," Moran told a reporter. "I just wish he was on my side."

But another GOP senator sees a very different McCain. This senator, who, like other McCain critics in the Senate, asked not to be quoted by name, calls him an "engaging rogue" who plays to the national news media, spreads himself too thin, angers too quickly and often rides roughshod over colleagues. "Once he's staked out a position, he is impossible to deal with," this senator added.

More than anyone else now in the Senate, McCain is an insider who remains an outsider at heart. As chairman of the Commerce, Science and Transportation Committee, McCain is one of the barons of the Senate, with a respectable but not unblemished record in winning passage of legislation dealing with telecommunications and the Internet as well as more traditional aspects of commerce.

As a member of the Armed Services Committee, with military experience that included 5 1/2 years as a prisoner of war in North Vietnam, he has criticized President Clinton's national security policies but refused to join other GOP conservatives in trying to curb his powers as commander in chief or cut off funding for controversial military missions. McCain fought unsuccessfully to authorize ground troops in Kosovo and successfully backed Clinton's decision to reestablish diplomatic relations with Vietnam.

But institutional power only seems to have whetted his appetite for grander causes that burnish his reformer's image but drive his colleagues crazy.

A case in point is his impassioned crusade against what he regards as unnecessary, politically inspired "pork barrel" spending. While he generally supports more money for defense, he has gone after military as well as domestic "pork," arguing it siphons funds from more worthy causes, such as military readiness and compensation for enlisted personnel.

Targets have ranged from the multibillion-dollar B-2 bomber and Sea Wolf submarine to $1.1 million for a manure disposal project and $750,000 for research on grasshoppers. He lists each such project in the Congressional Record and forces votes on some of them.

While he has not succeeded in reforming the spending culture of Congress, Senate Appropriations Committee Chairman Ted Stevens (R-Alaska)--himself a target of some of McCain's pork-busting efforts--says McCain has had a salutary effect.

When faced with colleagues, interest groups or constituents who are demanding more than he can deliver, Stevens tells supplicants they must be prepared to deal with McCain. Often it works. As an example, Stevens cited the case of an Alaska mayor with a 40-item wish list who, after McCain's name was invoked, settled for one project shared with a couple of other cities.

In McCain's mind, his biggest failure is the bipartisan effort he has helped lead for the last four years to overhaul the country's loophole-ridden campaign fund-raising laws. While the House has twice approved the legislation and a majority of the Senate has gone on record for it, McCain and co-sponsor Sen. Russell Feingold (D-Wis.) have failed to win the 60 votes needed to break a GOP-led filibuster against it--and appear likely to fail again when it comes up this week.

The 1998 bill to implement a settlement of lawsuits against the tobacco industry was another conspicuous defeat, although key players blame others more than McCain, including Republican leaders, tobacco lobbyists and public health figures who shunned compromise. McCain took over the bill at the request of GOP leaders, embracing it with characteristic zeal. He moved it out of committee only to see it implode on the Senate floor.

"He got sideswiped by the Republican leadership in a way that he could hardly have anticipated," said Matthew L. Myers, executive vice president of the Campaign for Tobacco-Free Kids.

But McCain faults himself too. "We were driven by [former surgeon general C. Everett] Koop and [former food and drug commissioner David A.] Kessler and organizations that said we couldn't give up on anything. It's a lesson I've learned for campaign finance reform: You can't let the perfect be the enemy of the good." This was a reference to his decision to concentrate on banning unregulated "soft money" donations to political parties, dropping other contentious provisions for the time being.

Despite his apostasy on some issues, McCain, who votes against abortion rights and for tax cuts, ranks high on ratings by conservative groups and in party loyalty rankings. Environmentalists applaud his record for Arizona but find fault with his national environmental record. Consumer groups agree with his vote against the 1996 Telecommunications Act but disagree with his reasons: McCain wanted more deregulation; the groups wanted more regulations to protect consumers.

McCain's career as a political reformer has come a long way since his first few months in the Senate in 1987, when he joined four other senators in meeting with federal regulators on behalf of Charles H. Keating Jr., a high-flying savings and loan executive who became embroiled in a long legal battle over allegations of fraud. It was an intensely painful ordeal that McCain has likened to his torture as a POW in Hanoi. In the end, the Senate ethics committee rapped McCain's knuckles for faulty judgment, while punishing three of the others more severely.

More recently, he has taken some hits from other Republicans for pushing campaign finance reform while raking in money for his campaign from special interests, including telecommunications and transportation interests that have a stake in legislation before the commerce committee. It "rings a little hollow," Majority Leader Trent Lott (R-Miss.) complained recently of McCain's efforts.

CAPTION: Sen. John McCain, right, and Rep. Asa Hutchinson discuss campaign finance reform at a news conference.