Sen. John McCain, the Arizona maverick and former Vietnam prisoner of war, formally launched his uphill bid for the Republican presidential nomination here today calling for a "new patriotic challenge" to reform the political system and strengthen the nation's defenses.

McCain, who has emerged as a credible underdog challenger to Texas Gov. George W. Bush, implicitly drew a contrast between their life experiences, saying his ordeal in a Hanoi prison taught him "both the blessing and the price of freedom" and gave him the confidence to make his own judgments on issues of war and peace.

Bush has acknowledged needing tutelage on national security issues, and after questions about his service in the Texas Air National Guard during the Vietnam War has been forced to deny that favoritism to his father, who was in Congress at the time, eased his way from Yale into that stateside unit.

McCain identified himself to a noontime crowd of about 1,000 at a park here as "the son and grandson of Navy admirals," a man "born into America's service. . . . It is because I owe America more than she has ever owed me that I am a candidate for president," he said.

With his compelling personal story and knack for staying in the news, McCain is perhaps as well-positioned to upset Bush as anyone in the race. His endurance through 5 1/2 years in North Vietnamese prisons is movingly recounted in his new best-selling autobiographical book, "Faith of My Fathers."

McCain has been raising about $2 million per quarter this year and "will have enough cash and matching funds to max out in the early states," said campaign manager Rick Davis -- though not enough to keep pace with Bush and Steve Forbes.

In his speech, McCain pledged to press for overhaul of the campaign finance system -- a battle on which he has been fighting his party's leaders for years -- and to veto any "pork barrel" spending bill that reaches him as president. He also offered voters tax cuts, pledged to seek a nationwide test of school vouchers and vowed to save "every single dollar" of the Social Security surplus.

But mostly, McCain used his announcement speech to begin drawing a personal contrast with Bush, who leads the field in fund-raising and endorsements, in part because he is the son of the former president. Without mentioning Bush by name, McCain said, "I don't begin this mission with any sense of entitlement." And he said America needs "the service of all of our children, not just the sons and daughters of a privileged elite."

McCain's military service was a key theme throughout the day. In the morning he had breakfast with the midshipmen at Annapolis, where he, like his father and grandfather, was educated. And in his speech here McCain spoke of the "most solemn responsibility" of the president -- as commander in chief. The 63-year-old senator still bears the scars from the beatings he received after his Navy fighter was shot down over Hanoi.

"When it comes time to make the decision to send our young men and women into harm's way," he said, "the decision should be made by a leader who knows that such decisions have profound consequences. There comes a time when a nation's leader can no longer rely on briefing books and talking points, when the experts and the advisers have all weighed in. . . . For no matter how many others are involved in the decision, the president is a lonely man . . . when the casualty reports come in. I am not afraid of the burden."

The three-term senator pledged to restore the military readiness he said President Clinton has squandered, but characteristically blamed "both parties in Congress [that] have wasted scarce defense dollars on unneeded weapons systems and other pork-barrel projects." One of his biggest cheers came when he said, "I will refuse to sign any pork-barrel bill that crosses my desk. And if Congress overrides my veto . . . I'll make sure you know who they are -- every one of them."

McCain made the decision this summer to ignore Iowa, whose caucuses come eight days before the Feb. 8 New Hampshire primary, in a calculated gamble that he can derail Bush's bandwagon here, where independents play a significant role in the GOP primary, and in South Carolina with its large community of military veterans.

New Hampshire GOP Chairman Steve Duprey said McCain has built an organization here "second only to Bush's." In South Carolina, where McCain will campaign Tuesday, local observers also rate him the main challenger to Bush.

Veteran GOP consultant Eddie Mahe said he thinks McCain "can pull off" skipping Iowa, "but only if Bush on his own screws up really badly or if Steve Forbes goes after Bush so tough that he raises real doubts about him." In that case, Mahe said, the beneficiary would likely be McCain or Elizabeth Dole, "and McCain would have the advantage, because he is a better politician."

McCain's battles against what he regards as wasteful spending and to pass campaign finance legislation have made him powerful enemies in GOP ranks. But Mahe said these issues play well with voters, saying, "To the extent he is a maverick, they say hooray."

Just as Kosovo provided an opportunity for countless TV appearances in which McCain displayed his military background, the coming Senate battle on campaign finance will offer him more air time to reinforce that maverick image.

The "new patriotic challenge" McCain outlined pits him "against the pervasive cynicism that is debilitating our democracy . . . a fight to take our government back from the power brokers and special interests."

Critics note that despite that rhetoric, McCain has enlisted a former chairman of Merrill Lynch as his finance chairman and was accompanied here by Kenneth Duberstein, who was the Reagan White House chief of staff and now heads one of Washington's most influential lobbying firms.

And McCain did not neglect a number of traditional Republican themes, pledging to slash the inheritance tax and fight teachers unions over school vouchers. Notably absent from his speech was any mention of abortion or other social issues. Despite a long record as an antiabortion legislator, McCain stirred controversy last month by implying he would not try to reverse Roe v. Wade.