Senator Bernie Sanders routed Hillary Clinton in all three Democratic presidential contests on Saturday, infusing his underdog campaign with critical momentum and bolstering his argument that the race for the nomination is not a foregone conclusion.

Mr. Sanders found a welcome tableau in the largely white and liberal electorates of the Pacific Northwest, where just days after resoundingly beating Mrs. Clinton in Idaho he repeated the feat in the Washington caucuses, winning 73 percent of the vote. He did even better in Alaska, winning 82 percent of the vote, and in Hawaii, he had 71 percent with a few precincts still be counted, according to The Associated Press.

Washington, the largest prize Saturday with 101 delegates in play, was a vital state for Mr. Sanders, whose prospects of capturing the nomination dimmed after double-digit losses to Mrs. Clinton across the South and weak showings in delegate-rich Ohio, Florida and North Carolina this month. As of Saturday evening, Mrs. Clinton had roughly 280 more pledged delegates, who are awarded based on voting, and 440 more superdelegates — party leaders and elected officials — than Mr. Sanders.