OLYMPIA — If control of the U.S. House comes down to any of the competitive congressional races in Washington 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 voting 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.