SACRAMENTO (AP) — The waiting is the hardest part.

With more than 830,000 primary ballots still uncounted, many candidates and campaigns in California remained on pins and needles Thursday awaiting the results of undecided races.

Proposition 29, the proposal to increase taxes on tobacco products to pay for cancer research, was among the contests that remained too close to call.

Election officials warned that more of the same could occur after November's general election, when the stakes are even higher, due to California's all-paper voting system and meticulous legal requirements for counties that tabulate results.

More than half of California voters now cast ballots by mail, requiring elections officials to verify signatures and voting status. Ballots delivered to polling places on Election Day cannot be verified and counted until after polls close at 8 p.m.

In addition, thousands more voters cast provisional ballots when their eligibility is in question, they move, or lose their vote-by-mail ballot.

"Our job is to ensure accuracy. It's not about the speed. We've become this 'I want it now' society and people are just going to have to wait," said Gail Pellerin, president of California Association of Clerks and Election Officials and the registrar of voters for Santa Cruz County.

"We want to make sure everything's accurate and correct," Pellerin said. "It's not a simple, easy process."

In 2010, nearly 7.3 million absentee ballots were mailed to Californians, but only about two-thirds were returned, according to the secretary of state's office. Many of those same voters likely showed up at the polls attempting to cast provisional ballots, which take even longer to count than regular ballots.

The secretary of state's office said that at least 830,000 ballots remained uncounted Thursday afternoon. Many county clerks were expected to update their totals Friday.

Ninety percent of ballots cast are counted by 8 a.m. the day after the election, but the final 10 percent can take three to four weeks to tabulate because of lengthy administrative procedures to ensure that nobody voted twice, said Kim Alexander, president of the California Voter Foundation, a nonpartisan voter education group. Clerks have 28 days after the election to certify results.

She said California election laws are written to make it as easy as possible to vote, including allowing voters to drop off ballots at any polling place in their county on Election Day. However, the bollots must then be transferred to the correct precinct.

The state has also returned to an all-paper voting system after serious failures of electronic voting machines that caused massive problems at polling places.

"The bottom line is that in providing more avenues for voters to vote, we've also created more work for elections officials when it comes to counting ballots. We're trading convenience for timeliness," Alexander said.

With likely more than 20 congressional and legislative candidates facing same-party runoffs in November and voters possibly deciding more than a dozen ballot initiatives, the waiting could be even longer. That election, which includes the presidential race, is expected to generate much higher turnout than the abysmal 25 to 30 percent estimated turnout for Tuesday.

Eleven of the state's largest counties reported a total of 800,000 uncounted ballots as of Wednesday, with about 4.1 million votes counted so far. The state has a total of more than 17.1 million registered voters.

Counting the ballots more quickly has a downside, noted Alexander: The increased likelihood of recounts, which take even longer and cost more.