By JEFF ZELENY
TAMPA, Fla. — Mitt Romney accepted the Republican presidential nomination on Thursday by making a direct appeal to Americans who were captivated by President Obama’s hopeful promises of change, pledging that he could deliver what the president did not and move the country from its worst economic downturn since the Great Depression.
In his address on the closing night of the Republican National Convention, Mr. Romney asked voters to consider whether their lives had improved over the last four years and urged them not to feel guilty about giving up on Mr. Obama. He left little doubt about his chief argument against Mr. Obama in the fall, saying: “What America needs is jobs.”
But Mr. Romney not only delivered a forceful critique of Mr. Obama, he also used the marquee speech of his presidential campaign to make a case for himself, summoning people from each chapter of his life to paint a humanizing portrait in hopes of helping voters see him with a trusting eye. The effort underscored how Mr. Romney’s path to winning the White House reached well beyond Republican activists gathered here.
“If you felt that excitement when you voted for Barack Obama, shouldn’t you feel that way now that he’s President Obama?” Mr. Romney said. “You know there’s something wrong with the kind of job he’s done as president when the best feeling you had was the day you voted for him.”
With 67 days remaining before Election Day, the presidential race has been essentially locked in place, with each side hoping to win over a small slice of the electorate that is still undecided in fewer than a dozen states that are the leading battlegrounds across the country.
The speech loomed as arguably Mr. Romney’s most important since he began acting on his presidential aspirations nearly a decade ago. It was an opportunity to present himself to Americans who are just now beginning to tune in to this campaign and to make the case against Mr. Obama, particularly to the people who voted for him.
“I wish President Obama had succeeded because I want America to succeed, but his promises gave way to disappointment and division,” Mr. Romney said. “This isn’t something we have to accept. Now is the moment when we can do something.”
In the hours leading up to his speech at the Tampa Bay Times Forum, Mr. Romney’s life story unspooled before the Republican delegates. Personal testimonials were intended to reshape perceptions about Mr. Romney that have hardened after a negative television advertising campaign from the president and his Democratic allies. Business owners, longtime friends, Olympic athletes and members of his Mormon faith offered personal stories in an effort to humanize Mr. Romney.
Bob White, a business partner at Bain Capital and a longtime friend, took to the stage to testify that Mr. Romney was a decisive leader and conscientious investor.
“When things went wrong, we would not blame others,” Mr. White said. “He took decisive action. Mitt never hesitated. He made the tough decisions, coalesced the team, and moved forward.”
The stories were intended to help build a fuller picture of Mr. Romney, who has been reticent to talk about his faith or his charity works. The stories filled a long portion of the program, but it was not televised, so it remains an open question how successful the effort will be.
With the speech, Mr. Romney closed out his party’s convention here and prepared for a quick shift of public attention to the Democrats, who will gather to formally nominate Mr. Obama to a second term next week in North Carolina.
The dynamics shaping the general election campaign, particularly the challenges facing Mr. Romney, came to life during the Republican convention and in his acceptance speech on Thursday night. Improving his standing among female voters is critical to his chances for victory, advisers said, and Mr. Romney amplified a steady theme of messages aimed at women.
“When my mom ran for the Senate, my dad was there for her every step of the way,” Mr. Romney said. “I can still hear her saying in her beautiful voice, ‘Why should women have any less say than men, about the great decisions facing our nation?’ “
A party that has struggled to increase its appeal to female, Hispanic and black voters featured a diverse lineup of speakers throughout the week, concluding with Senator Marco Rubio of Florida, who introduced Mr. Romney on Thursday evening. A favorite of Tea Party activists, Mr. Rubio embraced Mr. Romney as the right leader for the moment.
“Everywhere he’s been, he’s volunteered his time and talent to make things better for those around him,” Mr. Rubio said. “We are blessed that soon, he will be the president of the United States.”
The Republican convention, which was shortened by a day as Tropical Storm Isaac loomed, has been filled with sharp and forceful denunciations of the president and his policies over the last four years. Delegates cheered as speakers portrayed Mr. Obama as hostile to small-business owners, tolerant of increases to the national debt and out of touch with American values.
Former Gov. Jeb Bush of Florida received booming applause when he delivered a direct message to the president on Thursday evening, saying: “It is time to stop blaming your predecessor for your failed economic policies.”
“You were dealt a tough hand, but your policies have not worked,” Mr. Bush added. “In your fourth year of your presidency, a real leader would accept responsibility for his actions, and you haven’t done that.”
The Republican convention was teeming with excitement and optimism at the prospect of defeating Mr. Obama in November. The strong dissatisfaction with Mr. Obama helped swiftly push Republicans to rally behind Mr. Romney, who was long viewed with suspicion by many party activists over his evolving views on abortion and other social issues.
Mr. Romney is inheriting a party that is in generational and ideological transition — and that does not hold the affection for its presidential nominee that it did for, say, Ronald Reagan or George W. Bush.
The Republican Party is increasingly dominated by Tea Party conservatives who are pushing a platform of deep cuts in the size of government and for whom opposition to abortion rights and gay rights are almost consensus positions. But it is also dominated by divisions over tactics, as many Tea Party activists have rejected the politics of compromise that had been the way of life for many establishment Republicans.
Mr. Romney, speaking to an audience beyond the one inside the convention hall, sought to use the moment to rise above the bitter political sniping that has characterized much of the presidential campaign.
“The time has come for us to put the disappointments of the last four years behind us,” Mr. Romney said, according to the prepared remarks. “To put aside the divisiveness and the recriminations.”
Michael D. Shear, Adam Nagourney and Jonathan Weisman contributed reporting.