Mr. McCain’s advisers said they had put far less effort into the early voting program, instead sticking with what has worked for Florida Republicans for a decade: building up their margin with absentee ballots. But several Republicans said they were afraid that emphasis was missing the way voting behavior is changing here.

Mr. Obama has used sophisticated measures here to find and register new supporters. And Florida statistics this week, which sent a shiver of fear through Republicans, attest to his success: Democrats now have a 660,000 edge in voter registration over Republicans in the state, compared with a Democratic advantage of 280,000 voters in 2006.

Buzz Jacobs, the southeast regional manager of Mr. McCain’s campaign, suggested that Democrats would have trouble getting all those new voters to the polls. “They traditionally have a better voter registration system, and we have a better turnout operation,” he said.

But even several state Republicans said they saw evidence that Mr. Obama was bringing new and highly effective methods to the state to find voters and turn them out.

“I’ve gotten seven calls from live Obama volunteers  and the reason I’m getting calls is because I signed up on their Web site to get notifications from their campaign,” said Sally Bradshaw, a Republican who was a senior political adviser to Jeb Bush, the former governor.

Ms. Bradshaw, who supported Mitt Romney in the primary, had signed up for the list to keep informed about a rival. “I haven’t received any McCain calls,” she said.

Mr. McCain is in this spot today in part because of the conclusion by his campaign this summer that Florida, if competitive, was not as tough as it once was, and that there were more pressing states. Mr. Bush won here by five percentage points in 2004. The Democratic Party had earned months of bad publicity by pressuring its presidential candidates not to campaign in the state before its primary because Florida scheduled its vote earlier than party rules allowed.