Democratic presidential candidate former Vice President Joe Biden, accompanied by his wife Jill, speaks to members of the press.
Democratic presidential candidate former Vice President Joe Biden, accompanied by his wife Jill, speaks to members of the press at the National Constitution Center in Philadelphia, March 10, 2020. (AP Photo/Matt Rourke)

WASHINGTON - Former U.S. Vice President Joe Biden swept to victories Tuesday over Vermont Sen. Bernie Sanders in the Democratic presidential nominating contests in four states, including pivotal Michigan in the Midwest heartland of the country.

Biden, in his third run for the presidency over three decades, increased his lead in the race to oppose Republican President Donald Trump in November's national election with the wins in the U.S. auto manufacturing hub of Michigan, along with victories in Missouri in the Midwest, Mississippi in the South and Idaho in the West.

The victories make it ever more difficult for Sanders, a self-declared democratic socialist, to overtake Biden for the party's presidential nomination.

Democratic presidential candidate former Vice President Joe Biden speaks during a campaign rally at Renaissance High School in Detroit, Monday, March 9, 2020.

After a slow start in last month's voting, Biden told reporters in Philadelphia late Tuesday that he believes his campaign is "taking off," and he said Democrats will come together to retake the White House and unite the nation.

"I want to thank Bernie Sanders and his supporters for their tireless energy and their compassion. We share a common goal, and together we'll defeat Donald Trump," Biden said.

He and Sanders were both scheduled to hold Tuesday night rallies in Cleveland, Ohio, but canceled the events due to the coronavirus outbreak. Those cancellations may presage a dramatic change in campaigning practices, with both the Democratic candidates and Trump under pressure to limit the use of mass rallies to excite and motivate their supporters.  

Sanders did not make any public appearance Tuesday night. He won the vote in the small state of North Dakota in the Upper Midwest as the vote-counting continued Wednesday and also held a small edge in the western state of Washington, although the outcome there remained unclear.

The AP says former Vice President Joe Biden won the Democratic presidential primaries in Michigan, Missouri, Mississippi, and Idaho. Sen. Bernie Sanders won the North Dakota caucuses. The race in Washington state is still too close to call.

The two candidates are scheduled to debate each other Sunday ahead of next Tuesday's primary elections in Ohio, Florida, Illinois and Arizona. The Democratic National Committee announced there will be no live audience for the debate.

Based on pre-election polls, the results in Michigan, Missouri and Mississippi states had been expected, especially in Mississippi with its significant African-American population, a bloc of voters who have favored Biden in other states. In Missouri, voter exit polls showed that Biden also won the African American vote, as well as those who called themselves political moderates or conservatives.

Other exit polls showed Biden capturing the votes of blue collar workers, an important subset of the American electorate that Trump collected a sizable portion of in winning the 2016 election.

Michigan was the key prize among six states that voted Tuesday, with 125 delegates at stake to the Democrats' national presidential nominating convention in July.

The state could also prove pivotal in the presidential election since it was one of a handful of Midwest "rust belt" states Trump unexpectedly won four years ago on his path to a four-year term in the White House.

Biden entered Tuesday leading with 670 pledged national convention delegates so far, ahead of Sanders's 574 delegates, according to delegate trackers, with 1,991 needed to claim the nomination at the party convention. Pending final vote counts in the six states that voted Tuesday, Biden increased his lead in pledged convention delegates to at least an 843-681 edge over Sanders.

Exit polls of voters Tuesday conducted by the U.S. cable news network CNN indicated about half of Democratic voters in Michigan and Washington said they would trust Biden more than Sanders to handle a major crisis as president. In Missouri, the spread was wider, with about 60% saying they trusted Biden more in such a situation.

WATCH: Tuesday's results

The exit polls in Michigan and Washington also indicated Democratic voters considered themselves angry at the Trump administration.

The formerly long list of Democratic presidential candidates thinned markedly in the last 10 days, with challengers to Biden and Sanders dropping out of the contest just ahead of Super Tuesday voting a week ago or just after failing to win any states.

Gone from the race are former South Bend, Indiana, Mayor Pete Buttigieg, Minnesota Sen. Amy Klobuchar, former New York Mayor Michael Bloomberg and Massachusetts Sen. Elizabeth Warren. Another former candidate, entrepreneur Andrew Yang, endorsed Biden Tuesday night on CNN shortly after Biden sewed up his victory in Michigan.

"The math says Joe is our prohibitive nominee," Yang said. "We've got to start pulling the party together."