Associated Press

OLYMPIA, Wash. — Former Vice President Joe Biden won the statewide vote in Washington's vote-by-mail Democratic presidential primary, maintaining the narrow lead he's held over Vermont Sen. Bernie Sanders since last week as additional results were counted Monday.

Biden's victory means he won five of the six states that voted last Tuesday: Washington, Michigan, Missouri, Mississippi and Idaho. Sanders won North Dakota's caucuses.

In Washington, Biden led by more than 21,000 votes after more than 2.2 million votes were counted, and was winning in 25 of the state's 39 counties. Of the state’s 89 pledged delegates, only 31 are allocated based on the statewide result. The remaining 58 are determined based on the results of the state’s 10 congressional districts.

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Democrats are using the vote-by-mail presidential primary — moved up this year from May — for the first time to allocate delegates instead of the smaller caucuses used in previous years.

The Washington ballot had 13 candidates on it, along with an uncommitted option. But because almost all of the candidates had dropped out by last week, it was essentially a two-person race between Biden and Sanders. President Donald Trump was the only candidate listed for Republicans. Trump received more than 666,600 votes, and more than 11,000 Republicans wrote in an alternate candidate. More than 391,000 Democrats voted for candidates other than Biden or Sanders.

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In 2016, Sanders won more than two-thirds of the delegates from the Washington caucuses, which were attended that year by about 230,000 Democrats. More than 800,000 Democrats sent in ballots for the primary later that year — which Hillary Clinton won — even though their vote didn't count for the nominating process.

The state Democratic Party's central committee voted last year to start using a hybrid system that uses the state’s vote-by-mail system for a presidential primary to apportion delegates to candidates, and caucuses and conventions to select which delegates will represent the state at the national convention in Milwaukee in July. Also last year, Washington Gov. Jay Inslee signed a measure moving the state's presidential primary from May to the second Tuesday in March.