Former Secretary of State Hillary Clinton won Democratic primaries in Pennsylvania, Maryland, Connecticut and Delaware.
Sen. Bernie Sanders of Vermont notched a strong victory in Rhode Island, according to NBC, but Clinton's wins made his chances of becoming the Democratic general election standard-bearer slimmer than ever. The senator had already trailed by more than 200 pledged delegates heading into the day's contests.
"Certainly the math gets harder after tonight — that's clear just from the Maryland numbers," Sen. Jeff Merkley of Oregon, a Sanders supporter, told MSNBC on Tuesday night before all of the results came in.
And in her Tuesday night victory speech, Clinton struck a general election tone, hitting out several times at GOP front-runner Donald Trump and speaking of party unity.
"Whether you support Sen. Sanders or you support me: There's much more that unites us than divides us," Clinton said, pointing to Democrats' agreement that "wages are too low, and inequality is too high, that Wall Street can never again be allowed to threaten main street, and we should expand Social Security — not cut or privatize it."
Early exit polls painted a picture of the voter base in Tuesday's Democratic contests: The U.S. economy was the most important issue for 44 percent of respondents, followed by 22 percent saying income inequality and 20 percent pointing to health care, according to NBC News.
Keeping with the themes of their candidate's campaign, 78 percent of Tuesday Sanders voters said they believed that Wall Street hurts the U.S. economy. Only 51 percent of Clinton supporters agreed, according to early exit polls from NBC News.
Interestingly, 82 percent of Clinton supporters in Pennsylvania said they thought the ongoing Democratic campaign has energized the party, while only 58 percent of Sanders voters in the state said the same. A minority of Pennsylvania respondents behind both candidates said the race had divided their party — 37 percent for Sanders and only 12 percent for Clinton, NBC reported.
On gun control, 57 percent of Tuesday Democratic voters said they thought Clinton would better handle that issue, and 37 percent said Sanders, according to an early exit poll.
Clinton has led recent polls in the night's biggest Democratic prizes, Pennsylvania and Maryland, according to RealClearPolitics. She and Sanders also face off in Connecticut, Delaware and Rhode Island.
While Sanders' campaign has said it plans to reassess its standing after Tuesday's voting, Sanders maintains that he will stay in until the party's convention this summer, The New York Times reported Tuesday.
Sanders spoke relatively early on Tuesday night, just after polls were scheduled to close. Talking to a crowd in West Virginia, the Vermont senator gave a variant of his standard stump speech, but highlighted his competitiveness against Trump in head-to-head polls.
Meanwhile, Trump on Tuesday urged Sanders to run as an independent should he lose the party's nomination. This would presumably sap votes from Clinton in the general election.
In a written statement, Sanders implied that his candidacy was not necessarily about winning the party's nomination.
"The people in every state in this country should have the right to determine who they want as president and what the agenda of the Democratic Party should be," he said. "That's why we are in this race until the last vote is cast."
The next primary competition will come on May 3 when Indiana votes. Clinton has a slight lead over Sanders in recent polls in the state.