Earlier, Clinton had urged her supporters to back Barack Obama as the candidate.
“We were not all on the same side as Democrats, but we are now,” Clinton told a group of her supporters in Denver, Colorado.
“We are united and we are together and we are determined.”
A poll released on Monday showed 30 per cent of Clinton supporters would vote for John McCain, the Republican candidate, a third-party candidate or no-one.
In-depth coverage of the US election
McCain attempted to capitalise on allegations of splits within the party by releasing a campaign advertisement featuring a former Clinton supporter now backing him.
Carly Fiorina, a senior aide to McCain said disgruntled women supporters of Hillary Clinton “want a leader whose judgment and experience they can trust”.
Fiorina also said many Clinton supporters were “stunned” that Joe Biden, the senator who Obama has chosen as his vice-presidential candidate, was praised as a good debater and campaigner.
Obama, campaigning in Iowa, also attempted to play down a rift by saying Clinton had been on his vice-presidential short list.
“I am absolutely convinced that both Hillary Clinton and Bill Clinton understand the stakes,” he said.
“Hillary Clinton was clearly, in the Democratic primaries, the woman who … gathered the most votes – 18 million of them – was a great debater and a great campaigner,” Fiorina said.
Al Jazeera’s Rob Reynolds in Denver said Obama had work to do to win over Clinton supporters who felt she was treated poorly during the bitter primary election campaign between the two.
Obama, campaigning in Iowa, was reported to give a speech live via satellite on Monday, before formally accepting the nomination on Thursday.
The day’s events include a tribute to Edward Kennedy, the Massachussets senator and party icon now battling brain cancer.
Security in the city has been beefed up as more than 4,000 delegates and tens of thousands of officials, activists, protesters and journalists descend on the city.
The four-day convention will be capped with Obama delivering his acceptance speech before an expected 80,000 people on Thursday evening at Denver’s Invesco Field football stadium, on the 45th anniversary of Martin Luther King’s “I Have a Dream” speech – a highlight of the 1960s civil rights movement in the US.