I am going to be Democratic nominee: Clinton

Democratic front-runner Hillary Clinton has told her supporters that she would be the US presidential nominee from the party.

"I am going to be the Democratic nominee for President of the US," she wrote in an email to her supporters on Friday.

"And that means it's time to steel ourselves for the general election and a race against Donald Trump," she said.

Notably, Clinton is yet to reach the requisite number of delegates to be called as the Democratic Party's presumptive presidential nominee.

This is mainly because of the series of wins by her primary rival Senator Bernie Sanders from Vermont.

Sanders, however, asserted that he is not giving up and would be in the race till the last ballot is cast.

"Let me tell you also, so there is no mistake about it, we are in this fight until the last ballot is cast," he said.

"We acknowledge that it is an uphill fight, but it has been an uphill fight from the first day we were in this campaign," he told his supporters in New Mexico.

Sanders has notched wins in 20 states including three in the last three weeks in a row.

With nine contests still to come, including delegate-rich California, the Vermont senator is now backed by almost 46 per cent of the pledged delegates.

Clinton has commitments from 93 per cent of the so-called super delegates, top elected Democrats and party officials.

"These are the people who received no popular votes at all. I know many of these people. I like many of them. This is not an attack on them or their character," Sanders said.

He called it "a very bad idea" that more than 400 super delegates had lined up behind Clinton before any other candidate was in the race.

In his speech, Sanders said he is better placed to defeat Donald Trump in the general elections.

"Hillary Clinton is not the Democratic candidate most likely to defeat Donald Trump. Our campaign is the campaign," Sanders said, referring to the latest opinion polls.

Clinton has slipped behind Trump in two recent national polls and her edge over Trump narrowed significantly in a New York Times/CBS News survey published Sunday.

Sanders 13-point advantage over Trump was more than twice as great as Clinton’s shrinking six-point edge.

He cited a recent Fox News poll in which he said Trump "rather frighteningly" is ahead of Clinton by three points.

In that same poll, Sanders held a 46-42 per cent advantage over Trump.

"The reason we are the strongest candidate to defeat Donald Trump has everything to do with you. We have seen from coast to coast a level of excitement and energy in a campaign that we have not seen in a very long time," Sanders said.