Defeated Democratic presidential hopeful Hillary Clinton urged her frustrated followers to fall in line behind Barack Obama on Monday as a party rift threatened to rain on Obama's nominating parade.

Democratic Party chairman Howard Dean pounded a gavel to open a four-day convention that Democrats say Obama must use to unite Democrats, draw a sharp contrast with Republican rival John McCain and back up soaring oratory with a policy for leading the country.

"Looking out from this podium tonight, I see this diverse assembly of Democrats as a testament to the strength and unity of our party and the fruition of our 50-state strategy," Dean said.

The day's agenda was focused on outlining Obama's personal story. His wife, Michelle Obama, was due to give a keynote address on Monday night.

The son of a black father from Kenya and a white mother from Kansas, Obama, 47, spent formative years in Hawaii and Indonesia, worked his way through college and began his political career as a community organizer in Chicago.

Democrats hoped a tribute to Massachusetts Sen. Edward Kennedy, a Democratic Party icon battling brain cancer, would generate party enthusiasm. An aide to Kennedy said the senator would attend but not speak.

Casting a cloud over the convention was ongoing resentment from supporters of New York Sen. Clinton, miffed that she lost the nomination and upset that she was not picked as Obama's vice presidential running mate. Obama chose veteran Delaware Sen. Joe Biden, who arrived at the convention on Monday.

Clinton, speaking to sign-waving supporters from her home state delegation before the convention began, urged party unity.

"We are after all Democrats, so it may take a while," she said. "We're not the fall-in-line party. We are diverse. But make no mistake, we are unified," she said.

Amid reports that former President Bill Clinton was upset that he was asked to speak about foreign policy on Wednesday night instead of the U.S. economy, Obama told reporters travelling with him that he had told Clinton in a phone conversation last week he could talk about whatever he wanted.

"I said, Mr. President, you can say whatever you like. Bill Clinton is a unique figure in our politics," Obama said.

Obama also said in Moline, Illinois, that Hillary Clinton had been on his vice presidential short list. He played down a rift with the Clintons.

"I am absolutely convinced that both Hillary Clinton and Bill Clinton understand the stakes," he said.

A new opinion poll showed how much work lay ahead to rally Clinton supporters behind him. The CNN/Opinion Research Corp. poll said the race between Obama and McCain was even, each with 47 percent support.

It said 66 percent of Clinton supporters backed Obama, down from 75 percent at the end of June. Twenty-seven percent of them said they would support McCain in the Nov. 4 election, up from 16 percent in late June.

REHEARSAL

Michelle Obama took the stage in the half-empty hall along with her children and some family members early on Monday to check the layout ahead of her speech.

The hall's dominant feature is a giant, multi-tiered screen set behind the speaker that will project party logos and a film about Obama's life. Each delegation's seating space is marked with a vertical banner identifying its state.

Asked whether the convention would be harmonious, Michelle Obama said: "Yes, I am sure of that."

Obama spokesman Robert Gibbs said the choice for voters was between Obama and McCain, and noted that Clinton was publicly throwing her support behind the Illinois senator, who will deliver his acceptance speech on Thursday in a football stadium.

"If we put the choice in front of delegates here and voters all across this country, who's going to help rebuild this economy and put your family back to work, who's going keep your family safe at night, who's going to reduce our dependence on foreign oil ... The best person to do those things isn't John McCain, it's Barack Obama," Gibbs told CNN.

The McCain campaign has sought to exploit the Democratic divide. Senior McCain aide Carly Fiorina said disgruntled women supporters of Hillary Clinton "want a leader whose judgment and experience they can trust."

Fiorina said many Democratic women were "stunned" that Obama praised his vice presidential running mate, Sen. Joe Biden, as a good debater and campaigner "when Hillary Clinton was clearly in the Democratic primaries the woman who, the person who, gathered the most votes -- 18 million of them -- was a great debater and a great campaigner."