Obama himself was on the campaign trail on Sunday, in the battleground state of Wisconsin, greeting church goers in Eau Claire.
The Democratic candidate is running neck-and-neck with John McCain, his Republican rival, in most national polls ahead of the November 4 presidential election.
Democrats preached party unity on the eve of the convention, but anger among Hillary Clinton supporters flared again when Obama chose Biden as his running mate on Saturday, apparently without ever having investigated selecting the former first lady for the position.
Obama advisers have tried to play down the potential discord. Obama reportedly spoke with Clinton and her husband, Bill Clinton, the former president, this week and Clinton has been given a prime-time speaking slot at the convention on Tuesday night with Bill speaking on Wednesday.
Clinton is expected to meet her delegates on Wednesday morning and urge them to support Obama ahead of a roll call vote on the nomination that night, a Democratic Party source said.
But the Republicans were quick to fan the flames of a possible feud, launching a television advertisement featuring Clinton's criticism of Obama during the primary campaign.
|Security has been beefed up ahead of the convention in Denver [AFP]
"She won millions of votes but isn't on his ticket. Why? For speaking the truth," the narrator says, followed by Clinton's criticism of Obama's lack of specificity on issues and questions about his relationship to convicted Chicago businessman Tony Rezko, a former fundraiser for Obama.
A Clinton spokeswoman, however, said Clinton had expressed clear support for Obama.
"She has said repeatedly that Barack Obama and she share a commitment to changing the direction of the country, getting us out of Iraq, and expanding access to healthcare. John McCain doesn't. It's interesting how those remarks didn't make it into his ad," Kathleen Strand said.
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.