Hillary Clinton defeated Bernie Sanders in Delaware’s Democratic presidential primary Tuesday, in another demonstration of how difficult it is for the independent senator from Vermont to win so-called “closed” primaries where only registered Democrats are allowed to cast a ballot.

Clinton led Sanders by roughly 20 percentage points with over 95 percent of precincts reporting.

Clinton’s wins Tuesday in Maryland and Pennsylvania as well solidify her argument that she is popular in states across the country, rather than just a “regional” candidate, as Sanders’ campaign manager suggested after she swept the contests in the South.

Most of Delaware's top Democratic brass had endorsed the former secretary of state’s bid, including Gov. Jack Markell and Sens. Tom Carper and Chris Coons.

Delaware’s primary was a “closed” one, meaning that unaffiliated voters or independents could not participate. Sanders has won states with open primaries, like Michigan, New Hampshire and Wisconsin, but struggled in states with closed primaries, like New York. The only state holding a semi-open primary Tuesday was Rhode Island, which Sanders won; the day’s other primaries in Connecticut, Maryland and Pennsylvania were closed.

Though Delaware has less than a million residents and sends just 31 delegates to the state’s convention this summer, Sanders and Clinton both still visited Wilmington, the state’s biggest city. Sanders held a rally Saturday while Clinton hosted a get out the vote event Monday -- during which she gave a shout out to Vice President Joe Biden and his late son Beau Biden, the former Attorney General of Delaware, who died of cancer last year.

"I want to mention someone who’s not here but whose representation and advocacy for this state and for our country is legendary, and that is Vice President Joe Biden," she said. "And the Vice President’s distinguished record of service and his love of Delaware and his passion for our country is truly inspiring. And we lost an extraordinary young public servant when we lost Beau Biden, and that family has done so much for so many people, and I am just grateful to be in this state and be able to say what’s in my heart about the Biden family because it really is an extraordinary family of service and caring, and thank you, Delaware, for sharing them with us."

Only one poll of the state’s voters had been conducted before the primary. The April survey found Clinton leading Sanders by seven points, with 17 percent of likely Democratic voters still undecided.

Clinton had captured 1,944 delegates heading into Tuesday’s primaries, a number that includes superdelegates, compared to Sanders’ 1,192. If superdelegates are taken out of the equation, Clinton leads Sanders, 1,428 to 1,153. Either candidate would need 2,383 delegates to secure the nomination.

While Sanders admitted Tuesday that it would be “very hard” for his campaign to win if he has fewer pledged delegates when the primaries conclude, his campaign has argued that it could still bag the nomination by persuading superdelegates backing Clinton to switch their allegiances. Such a maneuver would go against the popular vote -- Clinton leads overall by 2.4 million votes so far -- and contradict the wishes of progressives backing his bid.