Mr Sanders still faces a steep climb to overtake Mrs Clinton but the big victories in the West generated more momentum for his upstart campaign and could stave off calls from Democratic leaders that he should wrap up his bid in the name of party unity.
Mr Sanders appeared headed to victory margins of more than 50 percentage points in Alaska and Washington, and led by about 40 points in Hawaii with some 90% of the results tallied there. “We are making significant inroads in Secretary Clinton’s lead and... we have a path to victory,” Mr Sanders told cheering supporters in Madison, Wisconsin. “It is hard for anybody to deny that our campaign has the momentum.”
Mrs Clinton, the former secretary of state, has increasingly turned her attention toward a potential November 8 general election showdown against Republican front-runner Donald Trump, claiming she is on the path to wrapping up the nomination.
Heading into Saturday, she led Mr Sanders by about 300 pledged delegates in the race for the 2,382 delegates needed to be nominated at the party’s July convention in Philadelphia. Adding in the support of superdelegates — party leaders who are free to back any candidate — she has 1,690 delegates to 946 for Mr Sanders.
Mr Sanders, a US senator from Vermont, needs to win up to two-thirds of the remaining delegates to catch Ms Clinton, who will keep piling up delegates even when she loses under a Democratic Party system that awards them proportionally in all states.
“These wins will help him raise more funds for the next few weeks but I don’t think it changes the overall equation,” said Democratic strategist Jim Manley, a Clinton supporter. “Hillary Clinton has too big a lead.”
But Mr Sanders has repeatedly said he is staying in the race until the convention, pointing to big crowds at his rallies and high turnout among young and first-time voters as proof of his viability. After raising $140m (€125m), he has the money to fight on as long as he wants.
He has energised the party’s liberal base and young voters with his calls to rein in Wall St and fight income inequality, a message that resonated in liberal Washington and other Western states. Mr Sanders won in Utah and Idaho last week.
After Wisconsin, the Democratic race moves to contests in New York on April 19 and a bloc of five states in the Northeast, led by Pennsylvania, on April 26.
There were no contests on Saturday in the Republican race featuring Mr Trump and rivals senator Ted Cruz of Texas and Ohio governor John Kasich.
On Saturday, the New York Times published a foreign policy-focused interview with Mr Trump.He told the Times he might stop oil purchases from Saudi Arabia unless they provide troops to fight the Islamic State. He also said he was willing to rethink traditional US alliances should he become president.