Hillary Clinton has called on US Democrats to unite behind Barack Obama as the party's presidential nominee, saying that despite a bruising primary battle, they were on the "same team".

"Whether you voted for me, or voted for Barack, the time is now to unite as a single party with a single purpose," the former first lady told a rally of Democrats on Tuesday night.

"Barack Obama is my candidate, and he must be our president," she said.

Clinton, a US senator for the state of New York, got a euphoric welcome as she arrived to give a prime-time speech on the second day of the Democratic National Convention in Denver.

Many in the audience held banners with the motto "unity".

"We are on the same team, and none of us can sit on the sidelines," she said, looking to appeal to supporters who remain upset at her narrow loss to Obama in the contest for the party's presidential nomination.

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"Let's be honest, this was a long, tough primary campaign, Hillary got 18 million votes, she won all the big states.  Everybody put their heart and soul into that."

But he said Clinton's speech marked the "the beginning of the end" of any political rift and would bring her supporters over to the Obama camp.

With the US presidential race set to ramp up after the Democratic and Republican conventions, Clinton said the campaign was "a fight for the future. And it is a fight we must win together."

Democrats, she said, had not "endured the last eight years" under George Bush to suffer more "failed leadership" from Republicans.

"No way, no how, no [John] McCain," she said, referring to the presumptive nominee for the rival Republican party.

Many Clinton supporters remain disgruntled at her primary loss [AFP]

"That was excellent, that was a strong speech. She made the case  for why we're going to be unified in November and why we're going to  win this election," he said, according to media reports. 

Allegations of tensions between the Clinton and Obama camps have cast a shadow over the Democratic convention, which will see Obama officially nominated as the party's presidential candidate.

On Tuesday, McCain released another campaign advertisement that sought to exploit alleged claims of a split.

The advertisement, the second in two days, revived arguments between the two Democratic rivals over national security policy during the primaries.

The advert reprised Clinton's comment during the primary battle that: "I know Senator McCain has a lifetime of experience that he will bring to the White House, and Senator Obama has a speech he gave in 2002."

"Hillary's right. John McCain for president," the advertisement concluded.