US Democrats are set for several more weeks of bitter infighting after Hillary Clinton's latest primary win ensured the battle for the party's presidential nomination will continue.

The former first lady claimed a groundswell of new support after defeating rival Barack Obama by 10 points in yesterday's pivotal vote, extending what is already the longest nomination race in US history.

But Democratic powerbrokers are under pressure to decide between the pair soon, bringing to an end their relentlessly negative campaigning, for the good of the party.

Mr Obama still leads the race in terms of delegates won, however neither candidate is likely to reach the key 2,025 figure needed to automatically secure the nomination.

Obama leads

That will mean the decision on which of them goes forward falls to the party's so-called superdelegates - the 795 elected officials and party bigwigs who may vote as they please at the nominating convention in August.

And they are being urged to hurry up and choose a winner, to avoid a divisive and very public last-minute scrap on the floor of the convention in Denver.

Mr Obama's supporters insist Ms Clinton cannot win.

"Senator Clinton would need 70 per cent of all the remaining pledged delegates to erase the lead in pledged delegates," said Obama campaign manager David Plouffe.

"We believe that on every measure - there have been 46 contests so far our record is 30 and 16, we have won twice as many states, we have a very significant pledged delegate lead - we do not believe that the structure of the race is going to change fundamentally," Mr Plouffe argued.

Clinton defiant

But the New York Senator remains defiant, refusing to back down and claiming she is the only candidate capable of beating Republican presidential hopeful John McCain.

"The delegates, all of them, have to make up their minds as to who is the stronger candidate. I believe in the last month I've demonstrated a real strength," she said.

"More people have now voted for me than have voted for my opponent. In fact, I now have more votes than anybody has ever had in a primary contest for a nomination," she told NBC.

All eyes are now on the next primaries, in Indiana and North Carolina on May 6.