WASHINGTON — Joe Biden decisively won Michigan's Democratic presidential primary on Tuesday, seizing a key battleground state that helped propel Bernie Sanders' insurgent candidacy four years ago.

The former vice president's victory there, as well as in Missouri, Mississippi and Idaho, dealt a serious blow to Sanders and substantially widened Biden's path to the nomination.

Biden again showed strength with working-class voters and African Americans, who are vital to winning the Democratic nomination. Sanders' narrow hopes for good news rested on North Dakota and Washington state. Washington's primary was too early to call, and because all votes there are cast by mail or by dropping them off in a ballot box, many ballots were marked for candidates who have since dropped out of the race.

Sanders led in Washington state by about 2,000 votes with about 70 percent of the votes counted. His lead in North Dakota was even smaller, about 500, with nearly 65 percent of the votes counted.

The six-state contest marked the first time voters weighed in on the primary since it effectively narrowed to a two-person race between Sanders and Biden. And the first four states on Tuesday went to Biden, a dramatic reversal for a campaign that appeared on the brink of collapse just two weeks ago. Now it is Sanders, whose candidacy was ascendant so recently, who must contemplate a path forward.

Addressing supporters in Philadelphia, Biden noted that many had "declared that this candidacy was dead" only days ago but "now we're very much alive." He also asked Sanders supporters to back him going forward.

"We need you, we want you, and there's a place in our campaign for each of you. I want to thank Bernie Sanders and his supporters for their tireless energy and their passion," Biden said. "We share a common goal, and together we'll beat Donald Trump.

It marked a high point for the former vice president's staff. They sipped beer and broke into an impromptu dance party after his speech, which was held close to his Philadelphia headquarters.

Even as the contours of the race came into shape, however, new uncertainty was sparked by fears of the spreading coronavirus. Both candidates abruptly canceled rallies in Ohio that were scheduled for Tuesday night. That set the stage for Biden's remarks in Philadelphia, while Sanders flew home to Vermont and didn't plan to address the public.

Sanders' campaign also said all future events would be decided on a case-by-case basis given public health concerns, while Biden called off a scheduled upcoming Florida stop. Still, the former vice president said Tuesday night that he'd be announcing plans to combat the coronavirus later this week.

The Democratic National Committee also said that Sunday's debate between Sanders and Biden would be conducted without an audience.

According to an Associated Press analysis, Biden had picked up at least 150 new delegates: 51 in Michigan, 40 in Missouri, 29 in Mississippi, five in North Dakota, 17 in Washington and eight in Idaho on Tuesday. Sanders got 88: 35 in Michigan, 23 in Missouri, two in Mississippi, six in Idaho, five in North Dakota and 17 in Washington.

Although six states voted, Michigan — with its 125 delegates — got most of the attention. Trump won the state by only 10,704 votes during the general election, his closest margin of victory among Michigan, Pennsylvania and Wisconsin. Those states gave Trump the narrow edge in the 2016 Electoral College after Hillary Clinton won the popular vote.

Sanders has vowed not to drop out regardless of Tuesday's results and frequently railed against the "Democratic establishment" that he says has aligned against him.

In addition to the powerful groups now siding with Biden, the former vice president has picked up the endorsements of many of his former presidential rivals, including Sen. Kamala Harris, Sen. Cory Booker and, as of Tuesday, entrepreneur Andrew Yang. Michigan Gov. Gretchen Whitmer, sometimes mentioned as a possible vice presidential choice, also endorsed Biden and campaigned with him ahead of Tuesday's primary.

Biden also gave a nod to all his former competitors, saying, "We're bringing this party together."

"That's what we have to do," he said.

Not every Democrat was lining up behind Biden, though. Michigan Rep. Debbie Dingell, who represents a sprawling district from the college town of Ann Arbor to the Detroit suburbs, said Tuesday that she's staying neutral.

"I remember what it was like four years ago and the vitriol and the anger, the people mad at each other the whole election cycle. We can't afford that," Dingell said. "It's about getting out and voting in November."

Like Sanders, Biden has no public events scheduled for Wednesday. And though he's celebrating a growing delegate lead, he's still confronting voters who question his positions, which include a gun control plan that reinstates an assault weapons ban and includes a voluntary buyback program for assault weapons.

That issue was at the center of a testy exchange with a worker while Biden was rallying earlier Tuesday in Detroit. The man accused him of "actively trying to end our Second Amendment right." Biden shot back, "You're full of s--t," but went on to say that while he supports the Second Amendment, "Do you need 100 rounds?"

Associated Press writers Mike Householder in Detroit and Seth Borenstein in Washington contributed to this report.