DENVER — Senator Obama of Illinois is signaling that he will meet the assault head-on and return fire of his own when Republicans and their presumptive presidential nominee, Senator McCain of Arizona, attempt to use their upcoming convention to fuel perceptions that Mr. Obama is out of touch with working Americans and too inexperienced to be trusted with the nation’s security.

“If John McCain wants to have a debate about who has the temperament and judgment to serve as the next commander in chief, that’s a debate I’m willing to have,” Mr. Obama said last night, accepting the Democratic presidential nomination before a crowd of 75,000 in the highly unusual venue of an outdoor football stadium.

Mr. Obama invoked one of the hallmarks of his campaign, his opposition to the war in Iraq, as he painted Mr. McCain as a key supporter of the strategy that put America’s military focus on Baghdad while Al Qaeda regrouped in Afghanistan.

“John McCain likes to say that he’ll follow bin Laden to the Gates of Hell — but he won’t even follow him to the cave where he lives,” the Democratic nominee declared. “You don’t defeat a terrorist network that operates in 80 countries by occupying Iraq. You don’t protect Israel and deter Iran just by talking tough in Washington.”

He repeatedly referred to the Republican as out of touch, saying: “It’s not because John McCain doesn’t care. It’s because John McCain doesn’t get it.”

Mr. Obama, the first African-American major-party nominee for president, noted that he was speaking on the 45th anniversary of the Reverend Martin Luther King Jr.’s “I Have a Dream” speech on the National Mall in Washington.

However, the Democratic nominee largely eschewed such soaring rhetoric last night, opting instead to recite a litany of problems faced by ordinary Americans. Students, soldiers, factory workers, retirees, and working women all heard their challenges embraced by the Democratic nominee. A film played before Mr. Obama took the stage underscored his modest roots and his stint organizing low-income neighborhoods in Chicago.

“This moment — this election — is our chance to keep, in the 21st century, the American promise alive. Because next week, in Minnesota, the same party that brought you two terms of George Bush and Dick Cheney will ask this country for a third,” Mr. Obama said. “We are here because we love this country too much to let the next four years look just like the last eight. … On November 4th, we must stand up and say: ‘Eight is enough.'”

The senator argued that his Republican opponent’s reputation as a maverick is largely undeserved.

“The record’s clear: John McCain has voted with George Bush 90% of the time. Senator McCain likes to talk about judgment, but really, what does it say about your judgment when you think George Bush has been right more than 90% of the time?” Mr. Obama said. “I don’t know about you, but I’m not ready to take a 10% chance on change.”

Soon after the senator of Illinois wrapped up his 44-minute address, a smattering of fireworks went off from Invesco Field and repeated salvos of red, white, and blue confetti and streamers were shot out over the stage. Mr. Obama’s wife, Michelle, and their daughters joined him, as did the vice presidential nominee, Senator Biden of Delaware, and his wife. The celebration will surely be mocked by Republicans as premature, but given the huge crowd anything less festive might have seemed like an anti-climax.

While the sense of history being made was palpable last night, it’s increasingly uncertain whether the groundbreaking nature of Mr. Obama’s candidacy will propel him to the presidency or perhaps make his goal more difficult. In recent weeks, despite the sluggish economy and widespread anger towards Washington, his lead in national polls has ebbed. Mr. McCain may even have picked up a narrow edge in the battleground state seen as deciding the 2004 race, Ohio.

Republicans, whose convention opens Monday in St. Paul, are promising a relentless focus on Mr. Obama’s lack of experience and on suggestions that he is an elitist. A Republican who managed Senator Dole’s presidential bid in 1996, Scott Reed, said the Democrats made a serious mistake by allowing stories about Senator Clinton and President Clinton to dominate three-quarters of their gathering. “They squandered three nights of their convention,” Mr. Reed said. “I assure you the Republicans are not going to do that.”

In one of yesterday’s warm-up speeches, Vice President Gore alluded to his own defeat in 2000 as he made the case for electing Mr. Obama. “Eight years ago some said there was not much difference between the nominees of the two major parties and it didn’t really matter who became president,” Mr. Gore recalled. “But here we all are in 2008 — and I doubt anyone would argue now that election didn’t matter.

Mr. Gore also complained that Mr. McCain, who has long proclaimed the dangers of global warming, recently backed away from a key measure to address the problem: caps on carbon emissions. “John McCain, a man who has earned our respect on many levels, is now openly endorsing the policies of the Bush-Cheney White House and promising to actually continue them,” the former vice president said. “The same policies all over again? Hey, I believe in recycling, but that’s ridiculous.”

Delegates also heard from an also-ran in the contest for this year’s Democratic nomination, Governor Richardson of New Mexico. He said Mr. McCain’s military credentials should not be a free pass to the White House. “John McCain served his country in war. We honor his service, but that doesn’t mean we have to make him president,” Mr. Richardson said.

The fired-up crowd made the event feel more like a outdoor summertime rock concert than the final night of a political convention. In fact, at times, it was literally a concert, as Sheryl Crow, Stevie Wonder, and others took to the stage to perform.

The concert-like air prevailed outside as well, where signs of ticket scalping abounded. Dozens of people seeking tickets loitered near the pathways to the stadium. One woman appeared to think invoking Mr. Obama’s message might be improve her chances. “Share the hope and any extra tickets,” her sign said.

In recent days, Republicans mocked the design of the stage Mr. Obama was to speak from, saying it resembled a Greek temple and underscored their view that the Democratic nominee is the object of an unseemly cult of personality, but last night the backdrop looked more like the exterior of the White House. However, Democrats pointed out that Mr. Bush accepted his nomination in New York four years ago in front of a stage decorated with similar columns.

Mr. Obama’s aides insisted that the oversize crowd was a boon, and they even put it to work before the speech last night, setting up a phone bank in one of the concourses to make calls to voters.

“There’s over 50,000 people from around the country who are attending this convention because we opened it up to America. And of course a large number of people are here from the battleground state of Colorado,” Mr. Obama’s campaign manager, David Plouffe, told some of the early arrivals at the stadium yesterday. “And on the night of November 4, our Republican friends will look back and realize this night and this convention is one of the reasons we turned Colorado Obama blue.”