LOS ANGELES - For over 1 million Californians, the Nov. 4 election is over. That's because they've already voted.

A growing throng of early voters in the nation's most populous state - perhaps comprising half of all votes to be cast in California's general election - has stretched Election Day into weeks. Candidates who wait until the end to close the deal with voters will be too late.

"The election is not a one-day event anymore. It's a 30-day event," said veteran Democratic strategist Bill Carrick, who is spearheading Kennedy clan member Bobby Shriver's campaign for Los Angeles County supervisor.

The midterm elections are just over a week away and California is one of more than 30 states in which some form of advance voting is shaping the way campaigns must be conducted. In some rural areas of the state, 8 of every 10 ballots cast could come through the mail.

The strategy-shifting dynamics caused by early voting are coming into play in states such as Florida, where Republican Gov. Rick Scott faces a tough re-election challenge from Democrat Charlie Crist, and Iowa, where GOP Senate candidate Joni Ernst hopes to defeat Democratic Rep. Bruce Braley and pick up one of the six additional seats the party needs for a majority.

Just last week, first lady Michelle Obama was in Iowa City urging college students to vote early for Braley at campus polling places.

In California, where nearly all the early voting is done by mail, the number of voters who registered as permanent absentee has been on the rise. In the state's June primary, nearly 70 percent of ballots were cast this way.

Other states focus their early voting efforts on placing polling booths in strip malls or other convenient locations weeks ahead of Election Day.

The early voting trend has been underway for years in many states, as voters seek the convenience of filling out a ballot at their kitchen table, or near where they work, shop or take classes, rather than traveling to a polling place on a particular day and waiting in line.

Almost 129 million people voted in the 2012 presidential election, 35.8 percent of them before Election Day. In the 2010 midterms, when Republicans regained control of the House, roughly 3 out of 10 voters cast early ballots.

More than 4 million people mailed in ballots in 2012 in California, up from about 3 million two years earlier.

"Vote-by-mail in California used to be something reserved for people who had permanent disabilities, people who were out of town on vacation," said Paul Mitchell, vice president with Political Data Inc., a research powerhouse that helps campaigns identify and track voters.

Early voting has changed the timing of campaigns. Mitchell recalled a recent campaign that poured money into a last-minute TV ad buy. "They were advertising basically to ghosts, voters who had already voted," he said.

For both major parties, finding and locking in early supporters has become essential.

The goal in the election remains the same, winning, but the playing field is longer. That means just about everything must start sooner and be maintained longer, whether TV and radio ads, phone calls to prospective voters or dispatching volunteers with clipboards to knock on doors.

Republican consultant Duane Dichiara, who is working on some hotly contested California legislative races, said the elongated voting period has increased the cost of campaigning and made it more challenging for campaigns to sustain fresh advertising messages for voters.