Democratic presidential candidate former Vice President Joe Biden speaks on stage after declaring victory in the South Carolina presidential primary on February 29, 2020 in Columbia, South Carolina.

Former Vice President Joe Biden is the apparent winner of Washington's Democratic primary, NBC News reported Monday, chalking up another victory in a string of recent upsets over Sen. Bernie Sanders, I-Vt.

With 99% of results in, Biden led with about 38% of the vote, edging out Sanders at 36%. Biden will win at least 42 of the state's delegates, while Sanders will receive 40, according to NBC.

Washington, which has 89 delegates up for grabs, was seen as a necessary pickup for Sanders following Biden's astounding gains in the Super Tuesday elections. Biden continued with his momentum the following week, with relatively easy wins in Michigan, Missouri and Mississippi.

But the results of Washington's primary were too close for NBC to call until days after voters cast their ballots on March 10, as the Evergreen State worked to combat the spread of the novel coronavirus.

Washington has more confirmed coronavirus cases than nearly any other state in the nation, and was the first to announce fatalities from the disease. Gov. Jay Inslee – himself a former Democratic presidential candidate – has overseen the state's response efforts, which include announcing school closures and working with the federal government to make testing kits more available.

Biden and Sanders, the last remaining major candidates in the race, laid out their own visions for responding to the outbreak during a debate Sunday night, as they vied for the chance to take on President Donald Trump in November.

Many political commentators had written off Biden's campaign as a failure just before his sizable victory in South Carolina, which was buoyed by a crucial endorsement from House Majority Whip James Clyburn, D-S.C.

Biden, who served as vice president under Barack Obama, rode that momentum to a wildly successful Super Tuesday, clinching a lead in delegates over Sanders and regaining his front-runner status virtually overnight.

That wave of primary elections washed many of the remaining candidates out of the Democratic field, which included two dozen contenders at its peak.

Three of the candidates – billionaire Tom Steyer, Minnesota Sen. Amy Klobuchar and former South Bend, Ind., Mayor Pete Buttigieg – threw in the towel on the eve of Super Tuesday. Multibillionaire and former New York Mayor Mike Bloomberg and Massachusetts Sen. Elizabeth Warren dropped out shortly after the 14-state primary day.

Klobuchar and Buttigieg, along with one-time 2020 candidate Beto O'Rourke, a former Texas representative, endorsed Biden the night before Super Tuesday, along with a slew of other Democrats.

The massive alignment behind Biden was viewed as a broad effort to defeat Sanders, a self-described democratic socialist who establishment Democrats fear would lose to Trump in a general election.

Polls showed Sanders as the favored candidate heading into the Washington primary, albeit by a much narrower margin than he had previously enjoyed. FiveThirtyEight's polling aggregate of the state gave Sanders a double-digit lead over Biden as recently as the end of February – but that lead shrunk to just about a six-point spread after Super Tuesday.

Still, Sanders remained a strong contender. He won overwhelmingly in Washington's 2016 caucuses, besting then-rival Hillary Clinton in every county and walking away with about 73% of the vote, according to data from The New York Times.

Mail-in ballots submitted prior to the primary accounted for more than a quarter of the total vote share, the Washington Secretary of State's website showed. The majority of those early votes were cast before Biden's turnaround, boding well for Sanders.

Washington was one of six states to hold primary elections March 10. Of them, only Michigan, with 125 delegates available, offered a larger reward for the remaining candidates in the race.

Idaho, Missouri, Mississippi and North Dakota also held primary elections that day.

On Tuesday, four delegate-rich states – including Florida, with its many swing voters crucial to a general election win – will hold their primaries. The other states are Illinois, Ohio and Arizona.

This is breaking news. Please check back for updates.

-- CNBC's Jacob Pramuk contributed to this report.