Hillary Clinton was projected to win Democratic primaries in Pennsylvania, Maryland and Delaware among the day's five East Coast contests, putting the presidential nomination within her reach after a long and bruising nominating contest.
Five states were voting: Pennsylvania, Maryland, Connecticut, Rhode Island and Delaware, but several locations in Baltimore stayed open an hour later because of earlier delays.
Clinton's rival Bernie Sanders won the Rhode Island primary.
According to exit polling compiled by the Washington Post, Clinton won 85 per cent of Maryland voters who identified experience as their top priority when picking a candidate.
Today's expected victories allow Clinton to reposition her campaign for the general election fight against Republicans in ways that have been difficult to do while fending off Sanders's persistent, well-funded and remarkably successful challenge.
Campaigning earlier today in Indiana, which votes on May 4, Clinton did not mention Sanders, although she repeated a line she has used often to suggest that Sanders's agenda is unrealistic and lacks specifics.
"It's not enough just to diagnose the problem," Clinton said after a tour of a steel plant in Hammond. "We all know that we are losing jobs. We all know that we are facing unfair competition."
"Give me the specifics - don't just give me the rhetoric and the demagoguery," she added.
She returned to Philadelphia, where she and former President Bill Clinton have campaigned heavily, for an election party.
Sanders has pledged to remain in the race, but a campaign strategist said they would assess his strategy going forward based on today's results. Sanders appeared to boisterous cheers in an arena in Huntington, West Virginia, choosing to hold a rally in a state that holds its primary in two weeks.
"This campaign is not just about electing a president. It is about transforming our nation," Sanders said at the outset of his remarks to a crowd of about 6400.
"It is about having the courage to demand a political revolution, and you are the revolutionaries."
He reminded the crowd that he has won 16 primaries or caucuses and argued that he is the better Democratic candidate to face Republican front-runner Donald Trump, who swept all five of the Republican contests.
Clinton's big victory in New York last week appeared to give her a boost in Pennsylvania, the state with the largest trove of 189 delegates. Sanders aides had once thought he could win there because the state shares demographic and economic characteristics with majority-white Midwestern states he captured earlier in the contest.
Sanders picked up Rhode Island, which had only 24 delegates at stake. Because Democratic delegates are awarded proportionately, a win in Rhode Island would likely yield Sanders a pickup of only a couple of delegates.
The large crowds that turned out for Sanders on Monday seem to have boosted the campaign's spirits about its prospects in Connecticut, which has 55 delegates in play.
Only 21 delegates are at stake in Delaware.
Throughout the race, Sanders has performed far better in states that allow independent voters to participate in their Democratic primaries. Only one of the five on the calendar today - Rhode Island - falls into that category.
Clinton has routinely outpaced Sanders among registered Democrats, while Sanders - who until his presidential bid had run as an independent throughout his political career - cleans up with unaffiliated voters.
Tad Devine, Sanders's senior strategist, said Sanders and his top aides plan to talk tomorrow about how his path to the nomination has been affected by today's results, but he said he sees no scenario in which Sanders drops out.
"Is our path to the nomination affected by five states voting? Yes," Devine said. "Is something dramatic going to happen? No, I don't think so."
In a fundraising solicitation sent about two hours before the polls closed, Sanders told supporters that "the political establishment wants us to go away so they can begin their march to the centre."
"Any victories and any votes we receive tonight, next week in Indiana, and in each state moving forward are a public declaration of support for the values we share," he said in the fundraising email.
"Our path to the nomination was never narrower than the day I announced my candidacy. I will not stop fighting for an America where no one who works 40 hours a week lives in poverty, where health care is a right for all Americans, where kids of all backgrounds can go to college without crushing debt, where there is no bank too big to fail, no banker too powerful to jail, and we've reclaimed our democracy from the billionaire class."
Sanders is poised to perform well in Indiana and could outright win other states voting soon, including West Virginia, Kentucky and Oregon. But there are no big prizes on the Democratic calendar in May that would allow him to capture a large number of delegates.
Clinton's lead among pledged delegates was above 200 before today's voting and she's now less than 300 delegates away from clinching the Democratic nomination if superdelegates are included.
There are just 13 Democratic contests left, including those in Puerto Rico and the District of Columbia, and Clinton is now 88 per cent of the way to reaching the 2383 delegates needed to clinch.
Even if Sanders were to score a blowout in California, which offers more than 500 delegates on June 8, Clinton appears certain to have locked up the nomination.
Clinton is ahead 46 per cent to 42 per cent in the latest Fox News poll from Indiana, with 92 delegates at stake. The next four contests offer a combined 171 delegates - not enough for Sanders to make much of a dent, since Democratic delegates are awarded proportionally.
A huge number of delegates are up for grabs on June 8 - 781 - but by then, Clinton is likely to need only a fraction of those to reach 2383.
Sanders is still outraising and outspending Clinton - and drawing crowds vastly larger than those she typically attracts.
- Washington Post, AP