SACRAMENTO  (AP) — With as many as half of California voters expected to cast their ballots by mail and several statewide contests narrowing to dead heats, Election Day has the potential to morph into election week.

The number of California voters casting mail-in ballots this year is expected to surpass 2008, when about 42 percent of the 13.7 million ballots cast in the presidential election were sent by mail. By comparison, 25 percent voted by mail in 2000.

The state distributed 8.9 million mail-in ballots this election cycle, about 20 percent more than were requested in 2008.

The rise in mail-in voting means that some of the highest-profile contests, from a statewide tax initiative to nationally watched congressional races, might not be decided by the time voters go to bed on Election Day if enough of those voters wait until the last minute to turn in their ballots.

"We've given people more avenues to vote, but to ensure there's no fraud and error, we have to take more time to verify the ballots," said Kim Alexander, president of the nonpartisan California Voter Foundation. "We've traded speed for convenience."

Absentee ballots take longer to count because elections workers must compare the signature on the mailed envelope with the one on that voter's registration card.

Many voters wait until the end to submit their ballots, which delays the process further, California Secretary of State Debra Bowen said.

"The biggest change is the number of ballots that are being dropped off on Election Day," she said. "Those ballots don't even start to be processed until 8 p.m."

California voters are most likely to drop off their ballots at polling stations during presidential contest years. In 2008, 3.2 million ballots— a quarter of the total vote count— didn't make the election night tally.

Many California races are likely to be nail-biters already, thanks to the success of a new independent process for drawing the state's legislative and congressional districts.

Several statewide ballot initiatives also are expected to be close, including Proposition 30, Gov. Jerry Brown's proposal to raise the statewide sales tax and increase income taxes on the wealthy to help close the state's budget deficit.

In the Central Valley, incumbent Democrat Rep. Jerry McNerney and Republican challenger Ricky Gill are prepared to wait days to know their fates. The redrawn 9th Congressional District is among the most competitive in the state.

"We're prepared for any contingency here, and that certainly could be one of them," said Gill, a 25 year-old law school graduate.

Lauren Smith, spokeswoman for the McNerney campaign, said a prolonged wait would disappoint supporters.

"It's an energy and excitement thing," she said. "It's a like Christmas Eve, and all of a sudden you're told Christmas is two days later."

The rise of mail-in voting likely contributed to the wait earlier this year for a verdict on Proposition 29, which would have raised the state's tobacco tax for the first time since 1998. The initiative on the June primary ballot lost by less than 1 percentage point during an election in which 65 percent of voters cast mail-in ballots.

The secretary of state's office will release the latest registration numbers Friday but said California appears to be on pace for a record number of registered voters. The state does not project turnout.

California saw the highest voter turnout for a presidential contest in 1964, when 88.4 percent of registered voters cast a ballot. The lowest was in 1996, when just 65.5 percent of voters weighed in.

County election officials are approving overtime and hiring extra workers to process the hundreds of thousands of mail-in ballots already starting to flood their offices.

Contra Costa County Registrar of Voters Steve Weir said he plans to triple his staff in the coming days.

"We'll have 80, 90 people working in every cranny of our warehouse," he said. "We're calling in people's children."

Still, registrars say Californians need to come to terms with the end of the era of knowing winners by bedtime.

"My job is to produce results that people are confident in, not to provide instant gratification for anyone," Shasta County Registrar of Voters Cathy Darling Allen said.