TAMPA, Fla. — Mitt Romney is heading into his nominating convention with his advisers convinced he needs a more combative footing against President Barack Obama, in order to appeal to white, working-class voters and to persuade them that he is the best answer to their economic frustrations.

Having survived a summer of attacks but still trailing the president narrowly in most national polls, Romney’s campaign remains focused intently on the economy as the issue that can defeat Obama.

But in a marked change, Romney has added a harder edge to a message that for most of this year was focused on his business and job-creation credentials, injecting volatile cultural themes into the race.

The Romney team intends to use the Republican convention, opening Monday, to lay out an aggressive case against the president and then to show economically pressed voters that Romney understands their struggles.

The convention will amplify a strategic shift that has been unfolding in recent weeks, reflecting a conclusion among Romney’s advisers that disappointment with Obama’s economic stewardship is not sufficient to propel Romney to victory on its own.

Republican strategists said that many middle-class voters had proved reluctant to give up entirely on Obama, and that they still needed to be convinced that Romney would look out for their interests.

Steven J. Law, the president of the conservative group American Crossroads, said some swing voters in focus groups had helped explain why support for Obama had not collapsed despite his poor marks on the economy.

“They’re somewhat seduced by the thought, `If the guy had more time, maybe he’d be able to turn it around,”’ said Law, whose group is spending tens of millions of dollars to change that.

Republicans are nervously monitoring the pivotal battleground of Ohio, where Romney has had trouble making headway against Obama, even as they see encouraging signs in Wisconsin and try to build up support among base conservatives and in the demographic groups already tending to back Romney.

Before supporters in Ohio on Saturday, Romney said, “He said he would do one thing, he did another.”

Accompanied by his running mate, he added, “We’re going to put in place someone who actually knows how to get the job done.”

Romney’s chances hinge to a large degree on running up his advantage among white voters in swing states who show deep strains of opposition to Obama but do not yet trust Romney to look out for them, Republican strategists say.

Many of those voters are economically disaffected, and the Romney campaign has been trying to reach them in recent weeks with appeals built around a message — not supported by the facts — that Obama is making it easier for welfare recipients to avoid work.

The campaign has also taken on a sharper edge with overtones of class and race. On Friday, Romney said at a rally that no one had ever had to ask him about his birth certificate, and his running mate, Rep. Paul D. Ryan, invoked his Catholicism and love of hunting. Democrats angrily said Romney’s remark associated him with the fringe “birther” camp seeking falsely to portray Obama as not American.

The convention will focus on a dual fire-Obama-hire-Romney message that will be on display here from Monday through Thursday as party leaders and delegates gather to kick off an intense, 10-week final stretch to the campaign.

Through videos, speeches and carefully staged programming, the convention — which could be threatened by an approaching tropical storm — will amplify what will constantly be described as Obama’s failures, with a focus on accusations that he has undercut struggling middle-class workers and small-business owners.

Some speakers are expected to use a softer, pragmatic approach, like former Gov. Jeb Bush of Florida, who is to address the delegates on Monday, while others will use a forceful hammer, like Gov. Chris Christie of New Jersey on Tuesday night.

Romney has run a television advertisement falsely charging that Obama has “quietly announced” plans to eliminate work and job training requirements for welfare beneficiaries, a message Romney’s aides said resonates with working-class voters who see government as doing nothing for them.

But with Ann Romney, Romney’s wife, taking the stage Tuesday night, the first of three nights to get broadcast network coverage, the Republican gathering will be as much about presenting Romney as a warm-blooded family man who understands the tribulations of everyday people. The campaign, after spending months arguing that the family’s Mormon faith was off limits, invited speakers from Romney’s church to testify how he had helped them when they were in need.

Those concurrent themes reflect a realization by strategists inside the Romney campaign and its allies at outside groups in recent weeks: Republicans need to do more than critique Obama’s economic record for Romney to win. With the race entering its final, decisive phase, strategists on both sides agree that Obama maintains a razor-thin edge.

That, several Republican officials said in interviews, is the result of a stubborn affinity for Obama among key swing voters who otherwise say they are disappointed in his job performance — a dynamic the Romney campaign and its allies are seeking to change.

Law said his group, Crossroads, had reserved roughly $35 million in advertising for the rest of the campaign and planned to spend more on efforts speaking to their other perception, that Obama had not been able to deliver.

“There’s a sequence to this,” he said, “which is voters have to conclude that it not only hasn’t worked, but it’s not going to work before they decide ultimately Mitt Romney is a suitable alternative.”

But, strategists acknowledge, Romney still has work to do before those critical swing voters will view him as that alternative, particularly with polls showing that voters see him as less attuned to their needs and values than Obama is. While he hopes to improve his standing among women, strategists say Romney’s chances hinge to a large degree on running up his edge among white voters who do not yet trust Romney.

“Right now the perceptions of him are allowing Barack Obama to stay in this race and keep a slight lead in spite of all the environmental factors that lead you to think he should be gone,” said Matthew Dowd, a pollster for George W. Bush’s campaigns. “If he can change perceptions about himself, then the environment takes hold, and if the environment takes hold, they win.”

Romney’s team is hoping to change perceptions starting with the Republican convention and, more important, with full access to the $186 million he and the Republican National Committee have on hand and can use as soon as Romney accepts the party’s nomination. It will give him his first real financial advantage over Obama this year.

Here and in Boston, Romney’s team is poised to sift through post-convention polling before pressing its new advantage with final advertising bets in key states.

“For undecided voters, Obama’s job performance weighs more heavily than Mitt’s current image,” said Neil Newhouse, the pollster for Romney. “They can measure what Obama has done, and his job performance numbers among those voters are extraordinarily weak.”

Central to the weeks ahead, strategists from both parties said, will be the perceptions of voters in battleground states like Florida, Iowa, North Carolina, Ohio, Virginia and Wisconsin. Both sides agree that Romney’s choice of Ryan has given Romney a new opportunity in Wisconsin. But, even Republicans say, the bigger electoral prize of Ohio as of now seems to be tilting in Obama’s direction.

With Crossroads and like-minded groups providing critical backup, Romney’s campaign is freer to concentrate on building its candidate up and trying to repair the damage done to his image over the summer.