Bernie Sanders looking for caucus hat-trick to narrow gap on Hillary Clinton

While Mr Sanders faces a steep climb to the nomination, a string of losses for former US secretary of state Mrs Clinton would highlight persistent vulnerabilities within her own party.

Mr Sanders continues to attract tens of thousands to his rallies, drawing more than 17,000 in Seattle this week, and has collected more than $140m from two million donors.

But turning that passionate support into the party nomination is growing increasingly difficult. Mrs Clinton has a delegate lead of 1,223 to 920 over Mr Sanders, according to an Associated Press analysis, an advantage that expands to 1,691-949 once the superdelegates, or party officials who can back either candidate, are included.

Based on that count, Mr Sanders still needs to win 58% of the remaining delegates from primaries and caucuses to have a majority of those delegates by the end of June.

His bar is even higher when the party officials are considered. He needs to win more than 67% of the remaining delegates overall - from primaries, caucuses and the ranks of uncommitted superdelegates - to prevail.

"I have gotten 2.6 million more votes than Bernie Sanders," Mrs Clinton told supporters crowded into a union hall in Everett, Washington, this week. "We are on the path to the nomination, and I want Washington to be part of how we get there."

On the Republican side, Donald Trump's latest rude comments about competitor Ted Cruz's wife raised new alarms among Republicans about the party front-runner's ability to win over women, especially in a potential autumn presidential match-up with Mrs Clinton.

Mr Trump came under fire for jabs at Heidi Cruz as the rivals engage in an increasingly bitter, personal battle for the party's nomination.

Hostilities reached a new high on Friday when Mr Cruz accused Mr Trump and "his henchmen" of stoking false rumours that he had cheated on his wife.

Meanwhile, Mr Sanders implored thousands of supporters in Spokane to come and see him speak again on Saturday - at a caucus. "Get there early," he said. "Let's have a record-breaking turnout."

On Tuesday, Mr Sanders won caucuses in Utah and Idaho but lost Arizona - the largest delegate prize - to Mrs Clinton. Because Democrats allocate their delegates on a proportional basis, meaning that the popular vote loser can still pick up a share, those victories netted Mr Sanders a gain of about 20 delegates.

He hopes to avoid a repeat of that performance by pulling out a win in Washington, which awards more than double the number of delegates than Hawaii and Alaska combined. He is also looking to contests that follow in Wisconsin on April 5 and Wyoming on April 9 as a way to build momentum.

Most of his dozen primary-season wins have been in states with largely white populations and in caucus contests, which tend to attract the most active liberal Democrats. He is heavily favoured by younger voters, who were a key part of the coalition that boosted Mr Obama to victory twice.

Mr Sanders dispatched his wife Jane to Alaska and Hawaii. Mrs Clinton, who held a conference call with supporters in Hawaii, did not send any high-profile supporters to campaign on her behalf.

Both candidates held several events in Washington state earlier this week.

Mrs Clinton has been looking past the primary contests and aiming at potential Republican challengers. In interviews, rallies and speeches this week, she largely focused on Tuesday's deadly attacks in Brussels, casting Mr Trump and Mr Cruz as unqualified to deal with complicated international threats.

Her campaign sees the April 19 contest in New York as an important one, not just because of the rich delegate prize but because losing to Mr Sanders in a state she represented in the Senate would be a psychological blow.

She hopes to lock up an even larger share of delegates in five Northeastern contests a week later.