FAIRFAX, Va. — About one-third of U.S. voters — a record number — could cast ballots for president before Election Day on Nov. 4.

Voting starts in earnest today. Statewide, residents of Virginia, Kentucky and Georgia can begin casting their ballots for Sen. Barack Obama, Sen. John McCain or other presidential candidates.

Experts predict up to a third of the electorate will vote early this year, up from 15 percent in 2000 and 20 percent in 2004. In closely contested Colorado, Nevada and New Mexico, about half the voters are expected to cast ballots before Nov. 4. Florida could be near to 40 percent.

"It's a sea change," said Rosemary Rodriguez, head of the U.S. Election Assistance Commission. "This is a little bit astounding."

It's all part of the most extensive early-voting process in history. The McCain and Obama campaigns are now focused on get-out-the-vote efforts even as their general election contest is taking shape, and before any debates have been held.

What used to be a 72-hour get-out-the-vote effort has expanded into what Rich Beeson, the Republican National Committee's political director, calls "a 720-hour program."

Each side says it's ready.

"We go through massive efforts to make sure our supporters know all the ways that they can vote," said Jon Carson, Obama's national field director.

The boom in early voting is also fed by election officials' desire to expand turnout without overwhelming polls on Election Day.

In Georgia, Secretary of State Karen Handel has urged voters to cast ballots early and expects about 1 million to do so — 25 percent of the electorate.

"We don't want voters to have to wait in line on Election Day," she said.

A couple of counties in Virginia and Kentucky allowed voting last week, but today residents can vote statewide. Both states only allow absentee votes by people unable to get to the polls on Election Day. Thirty-one states are more liberal, allowing anyone to vote early.

While Kentucky is likely to go for McCain, Virginia is competitive. That may be why turnout here in Fairfax County on Friday was higher than election officials anticipated. In all, 244 people voted on touch-screen machines. A day earlier, 96 people voted in Louisville, despite widespread power outages caused by Hurricane Ike.