Bernie Sanders won decisive victories in the Democratic caucuses in Alaska and Washington yesterday, giving his campaign a much-needed boost as he seeks to disrupt Hillary Clinton's path to the party's presidential nomination.

US networks projected Mr Sanders would win 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 were also facing yesterday in Hawaii, where 25 delegates are at stake. The results for that contest, where Mr Sanders was also favoured to win, are expected later.

Appearing at a campaign rally in Wisconsin, Mr Sanders said his commanding lead in Washington and Alaska, where voters turned out in high numbers, showed the tide was turning in his favour.

“We knew things were going to improve as we headed west," he said.

“We are making significant inroads in Secretary Clinton's lead and we have ... a path toward victory."

US networks projected Mr Sanders winning 79.2 percent against 20.8 percent for Ms Clinton in Alaska.

In Washington, he was projected to win 72.1 percent against 27.7 for Ms Clinton.

“This is what momentum is about," Mr 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 Mr Sanders in the trio of western states would inject momentum to his campaign as he seeks to dent Ms Clinton's lead in the race to their party's nomination.

Going into Saturday, Ms Clinton had already amassed 1,711 delegates, including super-delegates who are unelected by voters, compared to 952 for Mr Sanders, according to a CNN count.

To win the Democratic nomination at the July convention in Philadelphia, 2,383 delegates are needed.

Despite his victories yesterday, Mr Sanders, who has drawn strong support from young voters with his populist message, still faces an uphill battle to overcome Ms Clinton's lead, especially as Democrats allocate delegates proportionally by state.

Hillary Clinton still maintains the lead. (AAP)

Mr 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," he Sanders said to audiences at the 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 legalise 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, Ms 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 Ms Clinton that he needs to fill, Mr Sanders has refused to throw in the towel.

According to RealClearPolitics poll averages, in the remaining states with the three largest delegate allocations – California, New York and Pennsylvania – Ms Clinton leads Mr Sanders by nine points, 34 points and 28 points respectively.