ST. PAUL, Minn. — Senator McCain, a Vietnam POW turned political rebel, launched his fall campaign for the White House tonight in the final act of a Republican National Convention marked by extraordinary attention on running mate Sarah Palin.

Mr. McCain, 72 and hoping to become the oldest first-term president in history, faced a delicate assignment: presenting his credentials as a reformer willing to take on his own party and stressing his independence from an unpopular President Bush — all without breaking faith with his Republican base.

Aides earlier suggested he also would trace his career in public service — including more than five years as a Vietnam prisoner of war — while drawing stark differences with Democratic candidate Senator Obama. Among the conflicts: Mr. McCain deems the Iraq war essential to American interests, while Mr. Obama has called for a troop withdrawal.

Mr. McCain and Mrs. Palin arranged to depart their convention city immediately after the Arizona senator’s acceptance speech to fly to Wisconsin for an early start on the final weeks of the White House campaign.

The last night of the McCain-Palin convention also marked the end of an intensive stretch of politics with the potential to reshape the race. Democrats held their own convention last week in Denver, nominating Senator Biden as running mate for Mr. Obama, whose own acceptance speech drew an estimated 84,000 partisans to an outdoor football stadium.

The polls indicate a close race between Mr. McCain and Mr. Obama, at 47 a generation younger than his Republican opponent, with the outcome likely to be decided in scattered swing states in the industrial Midwest and the Southwest.

Ahead lie the traditional major checkpoints — presidential and vice presidential debates, millions of dollars in ads — but also the unscripted, spontaneous moments that can take on outsized importance in the race to pick a president.

The Arizona senator paid a brief visit to the Xcel center at mid-afternoon to check out a speaking podium remade overnight to capture the intimacy of a town-hall meeting that has become his trademark.

He was accompanied by his wife, Cindy, as well as two close allies, Senator Lindsey Graham of South Carolina and SenatorJoseph Lieberman, a Connecticut Democrat-turned-independent.

Mr. McCain’s wife and Mr. Graham also had speaking slots on the convention’s final night, as did former Pennsylvania Governor Tom Ridge, who had featured prominently in speculation about a running mate.

That was an honor that went unexpectedly to Mrs. Palin, the first female vice presidential candidate in party history, a 44-year-old Alaska governor virtually unknown nationally a week ago.

In the days since, she has faced a storm of scrutiny, some of it relating to her tenure as mayor of Wasilla, Alaska, and her time as governor, but most involving her 17-year-old unmarried daughter who is pregnant.

For the most part, Mr. McCain’s aides have kept Mrs. Palin out of public sight while vociferously defending her readiness to become president. She emerged Wednesday night during prime time to deliver a smiling, sarcastic attack on Mr. Obama that generated roars of approval — and acceptance — from the delegates.

She followed up in the hours before Mr. McCain’s convention appearance with a meeting with Republican governors and a fundraising appeal that blamed Democrats for spreading “misinformation and flat-out lies” about her family and her.

Even so, there were fresh questions about her readiness to sit one chair away from the Oval Office.

Mr. McCain has cited her authority over the Alaska National Guard as one example. But in a memo last spring, Air Force Major General Craig Campbell warned that “missions are at risk” in the state’s units because of a personnel shortage. The lack of qualified airmen, Campbell said, “has reached a crisis level.”

In an interview yesterday with The Associated Press, General Campbell said the situation has improved since then, but not enough to eliminate his concern that shortages will result in the burnout of troops.

Mr. McCain won the presidential nomination late last night in an anticlimactic vote that followed a campaign lasting most of a decade. He first ran for the White House in 2000, but lost the Republican nomination to Mr. Bush in a bruising struggle. He began the current campaign the Republican front-runner, but his chances seemed to collapse last winter when opposition to the Iraq war rose among independents and conservatives grew upset over his backing for legislation to give illegal immigrants a path toward citizenship.

In one of the most remarkable comebacks in recent times, he recovered to win the New Hampshire primary in early January, then wrapped up the nomination on Feb. 5 with big-state primary victories on Super Tuesday.

Mr. Obama, campaigning in swing-state Pennsylvania on today, said he wasn’t surprised at Mrs. Palin’s criticism of him, and said Democrats intended to focus on her record.

“I think she’s got a compelling story, but I assume she wants to be treated the same way that guys want to be treated,” he said. “I’ve been through this 19 months, she’s been through it — what — four days so far?”

Mr. Obama’s campaign announced it had raised roughly $8 million from more than 130,000 donors since Mrs. Palin delivered her speech last night.

Outside the hall, protesters calling for an end to the Iraq war vowed to march as Mr. McCain prepared to speak.

More than 100 demonstrators were arrested earlier in the day after a concert by the rock group Rage Against the Machine.

Police arrested more than 250 demonstrators on the convention’s first day on Monday, but the streets have been relatively quiet since.