Fresh from Democratic presidential primary wins in three US states, Bernie Sanders says he had political momentum that could help him win the backing of Democratic powerbrokers in his race against Hillary Clinton.

Senator Sanders easily won nominating contests in Alaska, Washington and Hawaii on Saturday.

The Vermont Senator's latest remarks reflect his plan to chip away at Mrs Clinton's commanding lead in the number of delegates needed to win the party's nomination for the November election.

Senator Sanders said Democratic "super delegates," who can change their allegiance, might face pressure to rally behind him because most polls suggest he has a better chance than Mrs Clinton of beating a Republican candidate.

"Momentum is with us," he told CNN.

"A lot of these super delegates may rethink their position with Hillary Clinton."

Clinton criticised over George Clooney dinner

Senator Sanders also criticised Mrs Clinton's reliance on wealthy donors to fund her campaign.

He cited a fundraising dinner being hosted next month by actor George Clooney where supporters will have to donate at least $US33,400 ($44,500) to attend.

Premium seating would require a $US353,400 donation — nearly seven times the US annual median income.

"It is obscene that Secretary Clinton keeps going to big money people to fund her campaign," Senator Sanders said.

"Our events, we charge $15 or $50 for people to come.

"So, it's not a criticism of Clooney, it's a criticism of a corrupt finance system."

Race for delegates tightens

About 85 per cent of the votes at the July Democratic National Convention in Philadelphia — where a party nominee will be chosen to face the Republicans in the November 8 election — are being determined by state nominating contests.

The other 15 per cent is held by party power brokers who are free to vote as they like, meaning they could hold the key in a tight contest.

After the weekend's contests, the former secretary of state led Senator Sanders by just under 300 pledged delegates in the race for the 2,382 needed to be nominated.

Overall, Mrs Clinton had the support of 1,712 delegates to 1,004 for Sanders, according to a tally by RealClearPolitics.com.

Senator Sanders is turning his attention to his native New York, where Democratic voters will divide up 247 delegates on April 19.

His campaign manager requested a debate in New York, which Mrs Clinton represented as a US senator for eight years, but no plans have been made.

Reuters