US networks projected Sanders winning by wide margins in both western states where 117 delegates were up for grabs -- 101 in Washington and 16 in Alaska.
The Vermont senator and the former secretary of state early Sunday were still awaiting the results of caucuses in Hawaii, where 25 delegates are at stake.
Sanders celebrated his Alaska and Washington victories via Twitter late Saturday.
"Thank you, Alaska! Together we are sending a message that this government belongs to all of us," he wrote.
"Washington, thank you for your huge support! It is hard for anybody to deny that our campaign has the momentum."
The delegate math, however, still dramatically favors Clinton who headed into Saturday's contests with a big lead among pledged delegates, and an even larger advantage when party officials known as superdelegates are factored into the equation.
Sanders however stubbornly maintains that he has a path to winning the nomination, and is plowing ahead state by state, starting with the next series of election contests next month.
Appearing at a campaign rally in the midwestern state of Wisconsin, which holds its primary on April 5, the Vermont senator said he feels the tide now turning in his favor.
"We knew things were going to improve as we headed west," he said to cheers.
"We are making significant inroads in Secretary Clinton's lead and we have... a path toward victory."
US networks projected Sanders winning 79.2 percent against 20.8 percent for Clinton in Alaska.
In Washington, he was projected to win 72.1 percent against 27.7 for Clinton.
"This is what momentum is about," Sanders told supporters in Wisconsin. "Don't let anybody tell you we can't win the nomination or win the general election. We're going to do both of those things."
A win for the 74-year-old Sanders in the trio of western states would inject momentum to his campaign as he seeks to dent Clinton's lead in the race to their party's nomination.
Going into Saturday, Clinton had already amassed 1,711 delegates, including super-delegates who are unelected by voters, compared to 952 for Sanders, according to a CNN count.
To win the Democratic nomination at the July convention in Philadelphia, 2,383 delegates are needed.
Uphill battle
Despite his victories on Saturday, Sanders, who has drawn strong support from young voters with his populist message, still faces an uphill battle to overcome Clinton's lead, especially as Democrats allocate delegates proportionally by state.
Sanders spent millions of dollars on campaign ads ahead of Saturday's caucuses and visited Seattle on Friday, giving a rousing rendition of his standard stump speech in which he railed against police brutality, a too-low minimum wage, soaring student debt and other ills.
"Real change historically always takes place from the bottom on up when millions of people come together," Sanders said to applause and cheers from the crowd in the city's Safeco Field baseball stadium.
"We need a political revolution!"
He repeated that same message on Saturday in Wisconsin -- the next state to hold primaries on April 5 -- and reiterated his vow to legalize marijuana.
"Everybody knows marijuana is not a killer drug like heroin," he said. "And that is why I have introduced legislation to take marijuana out of the controlled substance act."
By contrast, Clinton in recent days has already shifted her focus toward November's general election.
She delivered a somber counterterrorism speech Wednesday in the aftermath of deadly attacks in Brussels, using it as an opportunity to launch vigorous assaults on Republican candidates Donald Trump and Ted Cruz and warn their "reckless" foreign policies would harm US interests.
"We need to rely on what actually works, not bluster that alienates our partners and doesn't make us any safer," she said.
Despite the huge delegate gap with Clinton that he needs to fill, Sanders has refused to throw in the towel.
A series of recent polls has shown Sanders consistently doing better than Clinton against Republicans Trump, Cruz and Ohio Governor John Kasich.
Saturday's three contests were caucuses, essentially neighborhood meetings where voters can discuss political platforms and debate the merits of the candidates.
Since they generally require voters to show up in person rather than mailing primary ballots, the format favors Sanders, whose supporters have consistently shown more grassroots enthusiasm.
Millennials and first-time voters have been flocking to Sanders's message of economic equality, universal health care, and his call to reduce the influence of billionaires on the campaign finance system.
According to RealClearPolitics poll averages, in the remaining states with the three largest delegate allocations -- California, New York and Pennsylvania -- Clinton leads Sanders by nine points, 34 points and 28 points respectively.
