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If control of the U.S. House comes down to any of the competitive congressional races in Washington state and California, the nation might have to wait days to learn the outcome.

In both of those states, voters can put their ballots in the mail as late as Election Day, meaning the final votes do not typically reach election officials until several days later.

“I could see a scenario where we’re waiting a week or so to get results,” said Todd Donovan, a political science professor at Western Washington University. “The unfortunate thing is, you take longer and people are going to think something wrong is going on, when in fact, when you take longer, you’re more likely to have more accurate results.”

Washington is one of just three states that conduct all elections by mail, and it’s the only one of those three that allows ballots to be postmarked on Election Day. The system usually leaves about half of the vote outstanding at the end of the night, making it impossible to quickly determine the winner of close races.

In California, which has more than half a dozen competitive races in GOP-held districts, voters have the option of balloting by mail. Those ballots too must be postmarked by Election Day and received no later than three days after that. In past elections, some close California races have not been called for days. In its June primary, more than 67 percent of Californians voted by mail.

Ballots in Washington state will be mailed to voters next week. Three of the state’s 10 U.S. House races are being watched nationally as Democrats eye potential gains that could determine control of the chamber. The party needs a net gain of 23 seats nationwide to win back the House.

According to the National Conference of State Legislatures , 37 states and the District of Columbia offer some type of early voting, and 27 states and D.C. offer “no-excuse” absentee voting.

While more than 20 states allow certain elections to be held by mail, only Washington, Oregon and Colorado conduct all elections by mail. Colorado and Oregon both require that ballots — whether they are mailed or dropped off — be received by elections officials no later than Election Day in order to be counted.

Washington state’s 39 counties all post their initial results after 8 p.m. on election night. Many counties do daily updates after that, but because of the number of steps involved in ballot verification — including sorting, signature verification and assessment of ballots for extraneous marks — the updates can feel painfully slow for candidates locked in close races and the media organizations covering them.

“Fast is good, and I guarantee you that there’s nothing more that the counties want than to get the results out quickly,” said Washington Secretary of State Kim Wyman. “But it’s more important to make sure that they’re accurate.”

King County, by far the state’s most populous, is the largest of five counties voting in the 8th Congressional District, a closely watched open seat where Republican Dino Rossi and Democrat Kim Schrier are vying to replace retiring Republican Rep. Dave Reichert. Republican-held seats in the state’s 5th and 3rd Districts, in eastern Washington and southwestern Washington, are also being fiercely contested by Democrats this year.

Julie Wise, King County’s elections director, said she and her staff are ready for what will be a busy week.

“There’s no system that is perfect, but vote-by-mail is the strongest type of election system in the country,” she said. “We have a paper trail for everything.”

During the 2016 general election, more than 1 million ballots were cast in King County. The county posted results for over 615,000 ballots on election night, and about 30,000 the following day.

Wise said that a new tabulation system that came online in 2017 has helped increase the number of meaningful results they can post in the days after the election.

For the August primary, the county posted over half of the 557,000 votes cast on election night. With the new system, the day-after update increased to 45,000 ballots.

But it still takes about a day to go through the entire process, so the bulk of ballots that are brought in from across the county’s drop boxes on election night, in addition those still arriving via mail, will not be reflected immediately.

Even states like Colorado, where ballots must be in by Election Day, instantaneous results are not guaranteed.

Lynn Bartels, a spokeswoman for the Colorado Secretary of State, said that even with that firm deadline, mailed ballots require more time because of the procedures that need to be followed to process them.

“It’s not like you pull a lever and it all gets tabulated,” she said.

Washington’s system ensures that the state’s voters are not disenfranchised by mail delays and allows them to consider their options longer, Wise said.

“I think we have a better democracy when we have more people participating,” she said. “If we have to wait a day or two longer than others, I think that’s well worth it.”