PHILADELPHIA — Hillary Clinton added four more states to her victory column Tuesday night, strengthening what’s rapidly becoming an all-but-unstoppable march to the Democratic presidential nomination.

The Democratic front-runner expanded her sizable delegate lead with wins in Maryland, Pennsylvania, Delaware and Connecticut. But Bernie Sanders stopped her from sweeping the night with a win in Rhode Island.

Already, Clinton can lose every remaining primary by a wide margin and still capture her party’s nomination, according to an Associated Press analysis.

Still, the Vermont senator continues to attract tens of thousands to his rallies and raise millions of dollars online. He’s vowing to stay in the race through the last primary contest in June.

“We have a very narrow path and we’re going to have to win some big victories,” Sanders said in an interview with The Associated Press after an evening rally in West Virginia.

He must win 73 percent of the remaining delegates to clinch the Democratic nomination.

Clinton spent Tuesday in Pennsylvania and Indiana — two states her campaign believes could be critical in the fall election. After exchanging sharp barbs with Sanders earlier this month, she barely mentioned him in the run-up to Tuesday’s contests, underscoring her campaign’s growing confidence in her primary standing.

Her wins Tuesday put her fewer than 300 delegates away from clinching the Democratic nomination. She can reach that goal by winning just 21 percent of the remaining delegates and uncommitted superdelegates.

“We will unify our party to win this election and build an America where we can all rise together,” she told boisterous supporters at a rally in Philadelphia.

In exit polls conducted in Connecticut, Pennsylvania and Maryland, less than a fifth of Democratic voters said they would not support Clinton if she gets the nomination.

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In a town hall on MSNBC on Monday night, Clinton questioned the idea that she would need to adopt parts of Sanders’ platform to win over his supporters, saying that she did not make demands when she lost the primary to President Barack Obama eight years ago.

Sanders is not committing to easing Clinton’s path and said he has no plans to stop his attacks on Clinton. “Trust me, the Republicans have a very good research team,” he told AP. “They will go and take on Clinton in a way that I have chosen not, in areas I have chosen not to go.”

But there were signs that some of his supporters were beginning to accept that he might not make it all the way to the White House.

Charles Chamberlain, head of a liberal group backing Sanders, said the question isn’t whether the senator would win delegates. “It’s whether the Democratic establishment is going to bring our party together by embracing our fight,” Chamberlain said.

Democratic voters say the closely contested primary has excited the party. In Pennsylvania, about seven in 10 voters in Pennsylvania said the primary has energized the party rather than divided it, according to exit polls.

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Among Democrats, Clinton is 88 percent of the way to capturing the Democratic nomination with 2,097 delegates to Sanders’ 1,271. Those totals include both pledged delegates from primaries and caucuses and superdelegates, the party insiders who can back the candidate of their choice. It takes 2,383 to win the Democratic nomination.