DENVER — Senator Obama’s plan to address 100,000 supporters in a grandiose outdoor address at Invesco Field is drawing concern from supporters convinced the candidate needs a tightly focused pitch, not a coronation, to win over blue-collar voters throughout the industrial Midwest.

Tonight’s event, at which Mr. Obama will become only the second presidential nominee to formally accept the nomination in a sweeping open-air setting, runs the risk of putting off key swing voters instead of winning them over, backers and observers said. The festive atmosphere surrounding the speech, complete with planned musical performances by Stevie Wonder and the rap artist Will.i.am, could run contrary to the hard work and specific proposals for which undecided voters are looking. Seasoned political players cautioned privately and publicly that Mr. Obama faces a tough battle ahead, one that will require the full energy and attention of the candidate and his campaign team.

At stake tonight is whether Mr. Obama can stanch the bleeding among voters that has taken place during much of the summer. Mr. Obama has seen a larger lead in some polls, 12% in a June Los Angeles Times/Bloomberg survey, dwindle to an advantage of 1% to 2%, and, in the case of the Gallup poll, a 2-point disadvantage. Mr. Obama will have to convince voters that he has the experience and ability to meet the economic and international challenges the nation faces.

“It worries me,” a supporter of Senator Clinton now backing Mr. Obama, Lanny Davis, said of tonight’s setting. “Having rock star concert crowds and uplifting rhetoric doesn’t work with working-class voters. … Shots of 200,000 mesmerized people in Berlin didn’t help his campaign.”

While emphasizing that Mr. Obama could turn the corner and win the backing of important swing voters, Mr. Davis added: “He’s got hard work to do. He’s got to connect culturally and ideologically with working-class voters. And he can do it.”

A former ironworker and a congressman from South Boston, Mass., Rep. Stephen Lynch, said Mr. Obama had to do more work to win over blue-collar voters. “We’ve got to convince Democrats in the labor movement, especially in the building trades,” said Mr. Lynch, who is eager to campaign on the candidate’s behalf throughout eastern Pennsylvania and elsewhere in the Rust Belt. Such voters, Mr. Lynch, a former backer of Mrs. Clinton who voted for Mr. Obama yesterday, said, could be reached by direct, personal, and specific appeals from Mr. Obama, “not with fanfare.”

Asked about the Invesco Field setting, Mr. Lynch said it could backfire with senior citizens. “They’re not big with the celebrity stuff,” he said.

Mr. Lynch, who campaigned in 2004 for the Democratic nominee, Senator Kerry, suggested that Mr. Obama target blue-collar voters with a newfound intensity. Mr. Obama should “roll up his sleeves and go to some union meetings,” he said. “I think he can do it, but they feel he’s not spoken to them yet.”

A private pollster who formerly worked for Republican candidates, Frank Luntz, also said Mr. Obama’s plan tonight could hurt him. “He should be running stronger,” he said. “He’s running behind the core Democratic voter.”

The key demographic in the race is middle-income adults aged 40 to 59, Mr. Luntz said: “They don’t need Hollywood. They need answers.”

Beyond the issue of tonight’s speech, Mr. Obama’s supporters cautioned that the candidate had to sprint through the finish line. “It’s nip and tuck right now,” the mayor of Milwaukee, Thomas Barry, said. “He had an 8- or 9-point lead. … Now it’s a tight race.”

Mr. Barry’s explanation for Mr. Obama’s descent was simple: “During the month of August, McCain had better weeks than Obama.” He added, nonetheless, that he felt “very good” about Mr. Obama’s chances.

Mayor Nutter of Philadelphia, a key political player in the important swing state of Pennsylvania, minimized the tight polls. “It’s the late end of August. A whole lot of things are going to happen between [Election Day] and now,” said Mr. Nutter, who expressed optimism that voters would come out in large numbers for Mr. Obama in Philadelphia and its suburbs. “He’s going to get tremendously big numbers in Philly.”

A former pollster for President Clinton, Stanley Greenberg, said it was too early to worry about Mr. Obama. “Let’s see what happens when comes out of the convention,” said Mr. Greenberg, remembering Mr. Clinton’s predicament just prior to the 1992 Democratic National Convention in New York. “One has to have some perspective. Before our convention we were in third place. Half [of the voters] thought we didn’t have the honesty and character to be president of the United States.”