ST. AUGUSTINE, Florida — Mitt Romney sought to maintain his momentum in front of smaller crowds Monday, celebrating US victories in the Olympics and the space race after a weekend of super-sized events.

Romney had unveiled Paul Ryan, a deficit hawk congressman, as his vice-presidential pick on Saturday and campaigned with him for two days before the pair divided their efforts and the leader of the ticket headed to Florida.

There, in St. Augustine, the crowds lacked the electricity and numbers the pair drew in Virginia, North Carolina and Wisconsin before Ryan set off to Iowa to counter the effect of US President Barack Obama's visit there.

"I'm counting on you to help me win in November," Romney told a few thousand people, many of them retirees, at Florida's Flagler College.

He insisted the United States was still "the greatest nation on Earth."

"We just won more Olympic medals than any other nation," he said, a day after the Summer Games ended in London with the United States on top of the medals table.

"We just landed on Mars and took a good look at what's going on there," he added, referring to the Curiosity Rover operated by US space agency NASA at a cost of some $2.5 billion.

He also lauded America's 1960s missions to the moon and suggested that China, which plans to land a probe on the moon next year for the first time, "take a good look at our flag that was flown there 43 years ago."

The comments were followed by an election pledge to rein in spending by getting rid of programs that are not crucial. It was not clear if this includes the space program, which employs thousands of people in Florida.

Romney pivoted from space to the president's handling of the economy, alleging that Obama "promised everybody in this country the moon but he never got off the launch pad."

He cited slumping housing prices, particularly in Florida, millions of home foreclosures, and the "moral failure" of having 23 million Americans out of work, with unemployment at a stubbornly high 8.3 percent.

"Mr President, by your own measure you've failed to deliver the jobs Americans need," Romney said to cheers.

"We're tired of being tired of this country."

Romney was joined on the podium by Florida's freshman US Senator Marco Rubio, who continued the theme, branding November's coming election "a choice about our identity as a nation and as a people."

Many believe Rubio had been on the candidate's VP shortlist and, while the senator described Paul as "the next vice president of the United States," he did not dwell on the accomplishments of his rival.

Instead he focused on the contrasts between the Obama record and what the senator said would be a streamlined Romney administration with strong leadership at the top.

"Government is not the most important institute in society," Rubio said. "After all, what happens in your house is more important than what happens in the White House."

Florida's large number of seniors has made it a key focal point in the upcoming debate about the Romney-Ryan plans for entitlement programs like Medicare, the government health care program for retirees.

Ryan pledges to overhaul Medicare by converting it to a voucher system that will slash spending and require recipients to purchase health care on the open market, but critics say the system will leave seniors out of pocket.

The issue promises to be a major campaign debate, but seniors at the event were broadly supportive of Ryan's plan, despite its lack of details.

"The alternate solution that Mr Ryan has suggested is far smarter than existing Medicare," Kay Burtin, 83, told AFP.

"Putting it in the hands of a private insurer I think is the smart way to go. Senior costs will not rise because it will be competitive," she argued.

On Tuesday, Romney campaigns in Ohio while Ryan heads to Colorado, both among the roughly 10 swing states likely to decide the election.