Crist recalled the moment in his speech to the Broward crowd before Obama arrived and used the message to revive some classic Crist themes: bi-partisan cooperation.

“The president came here to help us,’’ he said. “One man got it done. President Barack Obama.”

“My friends this campaign, listen to me, is about optimism,” Crist said. “The other guys are pessimists.”

Obama and Romney entered the final 48 hours of campaigning on Sunday with electoral bravado about the certainty of winning mixed with urgent warnings for their most fervent supporters that the hard-fought race for the White House remains razor-close.

The two rivals started their day with rallies in the two still-competitive states where presidential campaigns begin every four years, and where the fates of their political futures could be decided: Iowa and New Hampshire.

Flanked by former President Bill Clinton in the shadow of the New Hampshire state capital building in Concord, Obama vowed to continue efforts to improve a recovering economy and expressed the confidence of an incumbent that voters across the competitive battleground states will give him the chance to try.

But he also betrayed the nervousness of a first-term president whose hopes for a second term — and the opportunity to continue shaping his legacy — hinges on a half-dozen states that could go either way by the end of election day on Tuesday.

“I am not ready to give up the fight and I hope you aren’t either, New Hampshire,” Obama said before thousands of people, his voice already growing hoarse at the start of a long day of campaigning. “We have come too far to turn back now. We have come too far to let our hearts grow faint.”

Romney spoke moments earlier with similar expressions of a certainty of success, telling about 4,400 supporters in Des Moines that the clock has nearly run out on the president’s time in office. He promised to usher in a new era of economic hope for families across the country who are struggling.

“Instead of building bridges, he’s made the divide between our parties wider,” Romney said. “Let me tell you why it is he’s fallen so far short of what he’s promised: it’s because he cared more about a liberal agenda than he did about repairing the economy.”

But like the man he wants to succeed, Romney is racing from swing state to swing state with the intensity of a candidate who recognizes that he is trailing — if only slightly — behind Obama in many of the states he must win to accumulate the 270 electoral votes he needs to become president.

The two candidates are set to hold a flurry of last rallies in the next two days to try to demonstrate the kind of enthusiasm among their supporters they hope will be evident at polling places on Tuesday.

But the candidate travel was only part of the battle on Sunday as their running mates, their campaign advisers and a myriad of other surrogates spread themselves out across the swing states and appeared on television to try to amplify the impact of the campaign message.

Rep. Paul D. Ryan, the Republican vice presidential nominee, is spending Sunday in Ohio, Minnesota and Colorado. Vice President Joe Biden began his Sunday campaigning with an event in Lakewood, Ohio, near Cleveland, and he was scheduled to campaign later Sunday in both Fremont and Lancaster, Ohio, before flying to Virginia, where he is scheduled to campaign in Sterling, Va., a Washington suburb, and Richmond on Monday.

The New York Times News Service contributed to this report.