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The campaign swing, which included stops Friday in the battleground states of New Hampshire and Iowa, is meant to build momentum coming out of the Democratic National Convention, where Mr. Obama accepted his party’s renomination on Thursday night. He’s motoring through Florida in Ground Force One, the black, million-dollar bus that the Secret Service unveiled in the summer of 2011.

Unemployment in Florida in July was 8.8 percent, higher than the national average. The president’s trip to Florida is his first since Mr. Romney and the Republicans held their convention in Tampa two weeks ago. With 29 electoral votes, the state is an important prize for both candidates. Prior to this weekend, Mr. Obama had already held 15 campaign events in Florida.

The latest polls show a tight race in the Sunshine State. A survey by Gravis Marketing last Sunday showed Mr. Romney leading by one percentage point, 48 percent to 47 percent. A poll by a Democratic-leaning firm a few days earlier found Mr. Obama leading by one point.

Mr. Obama is getting help from Florida’s former Republican governor Charlie Crist, who is campaigning with him and was a featured speaker at the Democratic convention.

Mr. Crist said called Mr. Obama “a leader that America needs.”

“We are blessed to have this president,” Mr. Crist told the crowd in Seminole.

The president’s bus tour will take him across the I-4 corridor in central Florida, a region with a fast-growing Latino population. Mr. Obama is holding rallies in counties that were key to his victory in the state in 2008.

Sen. Bill Nelson, Florida Democrat, called central Florida “the sweet spot” for candidates trying to win the state.

“If President Obama wins the state of Florida, with its 29 electoral votes, that’s the election,” Mr. Nelson said.

The president, who campaigned four years ago on a theme of “hope and change,” told supporters Saturday in Kissimmee, Fla., that a vote for him this year is still a vote for change. But he said progress is sometimes slow.

“Sometimes it’s frustrating, and change doesn’t happen the way it should,” he said. “Only you have the power to move us forward.”

This article includes wire service reports.