Former Vice President Joe Biden won the statewide vote in Washington's vote-by-mail Democratic presidential primary.

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 2.1 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.

President Donald Trump is the sole Republican on the ballot.

State Democrats are expected to make a decision in early April on which method they'll use to allocate delegates in 2020.