US presidential primaries: Hillary Clinton on brink of clinching Democratic nomination

Hillary Clinton stands poised to claim the Democratic mantle with tomorrow's primaries including California, overwhelming her rival Bernie Sanders and setting up a historic US presidential election showdown with Republican Donald Trump.

She is all but assured of locking in her party's nomination, a monumental step towards fulfilling a dream of returning to the White House as commander in chief, 16 years after serving as first lady to president Bill Clinton.

The former secretary of state neared the brink of victory with a strong win yesterday in the island territory of Puerto Rico a day after the US Virgin Islands voted overwhelmingly for her.

But a loss in California, the largest state in the nation, would blunt that momentum and potentially complicate Clinton's efforts to unify the party.

Clinching the nomination will launch an unprecedented battle between a candidate seeking to break the glass ceiling and become the country's first female commander in chief, and a provocative billionaire businessman and political neophyte who has changed the face of American campaigning.

Clinton, who like Sanders has stumped relentlessly in the Golden State in recent weeks, signalled the race has run its course and she will prevail as Democratic standard bearer.

"I believe on Tuesday I will have decisively won the popular vote and I will have decisively won the pledged delegate majority," she told CNN from California.

"After Tuesday I'm going to do everything I can to reach out to try to unify the Democratic Party and I expect Senator Sanders to do the same," she added.

But Sanders contends Clinton's lead is largely based on support from so-called super-delegates, party big-wigs who are not bound to vote for a specific candidate, and he proclaimed he will take his nomination fight all the way to the Democratic convention in July.

Sanders, a self-proclaimed democratic socialist, argued yesterday he could persuade many of the more than 500 super-delegates who back Clinton to reverse course and vote for him, adding that it would be "not quite accurate" for Clinton to claim victory by counting super-delegates before the convention.

Sanders has signalled he will use the time between the final Democratic primary -- June 14 in the US capital Washington -- and the July 25-28 convention to win over super-delegates.

A strong Sanders night tomorrow though, will not deny the inevitable mathematics of Clinton crossing the threshold.

She now stands at 2,354 total delegates, according to CNN's tally, just 29 shy of the number needed for victory.