TAMPA, Florida — Republican presidential nominee Mitt Romney took center-stage at his party's national convention Thursday, promising to restore what he called “the promise of America” if elected in November.

Romney made his case to be the next president before thousands inside the convention hall and millions more watching on television around the country. It was the biggest moment so far in Mitt Romney’s quest for the White House, and he moved quickly to offer a harsh critique of the man he would like to replace, President Barack Obama.

“America’s been patient. Americans have supported this president in good faith. But today the time has come to turn the page. Today the time has come for us to put the disappointments of the last four years behind us,” Romney said.

Romney said if elected he would focus on growing the economy, pledging to create 12 million new jobs and to make the United States energy-independent by 2020.

He also vowed to forge new trade agreements and warned that nations that cheat would face what he called “unmistakable consequences.”

Though much of his acceptance speech dealt with the economy, Mr. Romney did promise a different approach to U.S. foreign policy that would include strengthening ties with Israel and taking a tougher approach to relations with Russia, China and Iran.

“Every American was relieved the day President Obama gave the order, and Seal Team Six took out Osama bin Laden. But on another front, every American is less secure today because he has failed to slow Iran’s nuclear threat,” Romney said.

Romney concluded his address with an emotional pledge to build a better future that brought the Republican convention delegates to their feet. “That future is out there. It is waiting for us. Our children deserve it. Our nation depends on it. The peace and freedom of the world require it and with your help we will deliver it. Let us being that future for America tonight!,” Romnay said.

Romney was introduced by a rising star within the Republican Party, Florida Senator Marco Rubio. Rubio is of Cuban-American descent and is seen by some as a future presidential contender.

“Our problem with President Obama isn’t that he’s a bad person, OK? By all accounts, he too is a good husband and a good father and, thanks to a lot of practice, a good golfer. Our problem is not that he is a bad person. Our problem is he’s a bad president,” Rubio said.

Although this convention week in Tampa largely belonged to the Republicans, Democratic Party officials were on hand with a critique of the Republican nominee and his vice presidential running mate, Congressman Paul Ryan.

Democratic Party Chairwoman Debbie Wasserman-Schultz spoke to reporters near the convention site.

“No, we’re not focused or worried about what the Republicans are doing here. We’re certainly going to continue to make sure that the lies that the Republicans have told here, you know, the convention reinvention that Mitt Romney has attempted, which I don’t think will ultimately be successful,” she said.

Romney hopes his convention will give him a boost as he heads into the final stretch of the campaign against President Obama.

Mark Shields, an analyst for the NewsHour program on the Public Broadcasting System, says the presidential race is likely to remain close right into November because of the weak U.S. economy.

“You know I think the very fact that Mitt Romney, a flawed candidate, who is not emotionally engaged with the electorate up to this point, is basically running even with the president, who is far better liked, who is regarded as somebody who cares more about average people. I think it’s an indication that the dissatisfaction is a very salient and very persuasive factor,” Shields said.

The political spotlight now shifts to President Obama and the Democrats, who hold their convention next week in Charlotte, North Carolina.