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.
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.
"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.
Monday's key speech is set to be given by Michelle Obama, the wife of the Illinois senator, who will talk about her family's road to the presidential nomination.
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.