In Florida, however, Republicans appear to hold the upper hand, while in Colorado, early voting is about evenly split among Republicans and Democrats. Mr. Bush won all those states in 2004.

The dates when early voting begins and ends vary by state. Experts cautioned that the full impact of early voting cannot be known until the choices of those without party affiliations become more clear on Election Day.

In years past, however, early voting has tended to favor Republicans, according to voting experts. Mr. Bush won the early vote in 2004 in his race against Senator John F. Kerry, 60 percent to 40 percent. Mr. Bush won early voters by a similar margin in his 2000 run against Vice President Al Gore. As a result, the preliminary data from some states has surprised certain experts.

“In the past, what you’ve seen is early voters tend to be older, had higher incomes and lean more Republican and that trend has held over the past elections,” said Paul Gronke, executive director of the Early Voting Information Center at Reed College in Oregon. “But what we are seeing now when you look at the numbers is that they are more African-American, Hispanic and the young. I look at this and I go, ‘Wow!’ This is quite different. It is a lot different from what we’ve seen before and it has to raise concerns for the G.O.P.”

The early voting is part of a broader transformation in the way Americans vote. In the past, absentee voting was reserved mainly for those unable to make it to the polls on Election Day, whether because of sickness, business or military service. Now more than 30 states allow voters to cast early ballots either in person or by mail without requiring an excuse.