Democrats gathering here for their nominating convention are significantly more nervous about Barack Obama’s prospects this fall than they were a month ago, and are urging him to use the next four days to address weaknesses in his candidacy and lingering party divisions from the bitter primary fight.

Obama’s aides said they had learned from what they described as the mistake of the 2004 Democratic convention — when aides to Senator John Kerry’s campaign sought to forbid convention speakers from going after President Bush — and would use these four days to draw sharp contrasts with John McCain, particularly on the economy and McCain’s opposition to abortion rights.

“The stakes of this election will be made very clear,” said David Axelrod, Obama’s chief strategist.

At the same time, acknowledging persistent voter unease with Obama, his aides said they would use speeches and presentations over the next four days, including having Al Gore introduce Obama for his acceptance speech on Thursday night, to offer a fuller biography and a more detailed plan of what he would do as President.

In interviews, Democrats arriving here said they remained confident that Obama would leave Denver at the end of the week in a strong position to beat McCain. But many Democrats made clear that a convention they had once anticipated would be a breezy celebration of Obama had turned into a more sober and consequential event.

This reflected a summer that they said demonstrated Obama’s vulnerabilities and McCain’s resilience, and the signs of lingering divisions between some supporters of Hillary Rodham Clinton and Obama.

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