CHARLOTTE, N.C. — Four years ago, Barack Obama accepted the presidential nomination of a Democratic Party that was as unified and energized as at any moment in its history: Clintons and Kennedys, labor and Wall Street, centrists and leftists, old and young, blacks, whites and Hispanics.

It bristled with the excitement of history and the expectations of a new era.

But Democrats are arriving here to nominate Obama for a second term in an atmosphere far removed from the Denver convention in 2008, driven by a different kind of urgency and with new questions about their party's direction.

Their unity at this point is defined less by faith in Obama or a robust vision for what the party should stand for than by the prospect that Republicans could control the White House and Congress next year and enact a conservative agenda that would unravel much of what progressivism has stood for since Johnson's Great Society in the 1960s.

Mitt Romney's selection of Rep. Paul Ryan of Wisconsin as his running mate has only raised the ideological stakes for Democrats.

"It's because the Republican Party has moved completely to the right," Bob Kerrey, a centrist Democrat and former senator running again for the Senate from Nebraska, said of his party's newfound unity.

Caroline Kennedy, whose endorsement, along with that of her uncle, Sen. Edward Kennedy, of Obama in 2008 signaled the generational excitement that marked the last campaign, said Democrats were approaching 2012 in "a more serious sober way, given the conditions."

"They may not be as exhilarated as they were last time," Kennedy said. "But I think they are just as committed."

Invoking the conservative agenda as a foil is giving Obama's candidacy a more defined purpose, and campaigning against something can be a powerful tool for winning elections.

But it raises the question of whether, if he wins, Obama will be able to claim a mandate from voters and rally the elements of his party in the service of any kind of ambitious second-term agenda of his own.

"Certain interest groups in the Democratic Party who three months ago were complaining, 'They don't like this, they don't like that,' you don't hear that anymore," said Sen. Charles Schumer, D-N.Y. "The consequences of not winning are becoming clearer."

Unruly conglomeration

It is not as if the Obama coalition is tearing apart. In Washington, the White House and Democratic leaders in Congress have largely stuck together on legislative and political strategy, and at the grass-roots level there has been no ideological purity test of the sort that has fostered a series of disruptive primary challenges to established Republican incumbents.

Instead, the Democratic Party has in many ways reverted to form — an unruly conglomeration of sometimes competing interests, united in a belief that government has a crucial role to play in the economy and social justice, but often divided by priorities, means and values.

Moderate Democrats are concerned that Obama has alienated business constituencies by cracking down on Wall Street.

Occupy Wall Street is protesting in the streets of Charlotte over what it sees as too-lenient policies toward banks.

Liberals have criticized Obama for neglecting poverty and for continuing many of the anti-terrorism policies of President George W. Bush.

Centrists are open to cuts in entitlement programs to address the nation's long-term fiscal problems.

Liberals want more spending on education and health and higher taxes on the wealthy.

"I don't know if I've ever found anyone who has run for high political office who has been able to capture and hold every faction of the Democratic Party for long periods of time," said Tom Daschle, the former Democratic Senate majority leader. "There are some who wish he were more liberal and some who wish he was more conservative. He is what he is."

Even as he dealt with the aftermath of a financial crisis, Obama has built a strong legislative record, including his health-care bill.

He has, if slowly, taken stands that many members of his party long felt were politically risky, including support for same-sex marriage and issuing an order granting many young illegal immigrants brought to this country as children a two-year reprieve from deportation.

"They're Democrats — they are always going to be disappointed," Kennedy said. "But I think his stands on gay marriage and immigration this year changes that. I think he's accomplished some really substantive things that people gloss over."

A call for excitement

Still, some Democrats say Obama, consumed with crisis management in the early stages of his term and the rise of the tea party, has yet to rally the party to a long-term agenda that transcends the moment and would put his mark on the party beyond his presidency.

"For too many decades, we have been running on New Deal steam," said Gary Hart, the two-time presidential candidate and former senator from Colorado. "The principles are still here, but it's a whole new world out there."

Hart faulted Obama for some of this.

"He's had stronger opposition than any president in my lifetime," he said. "But there's still this feeling out in the country that he should rise above it. He's the president."

The evaporation of much of the excitement of 2008 — if perhaps inevitable, given the prolonged economic recession, partisan infighting in Washington and the reality of unmet expectations — has become a matter of concern for the White House in this competitive contest.

Bill Richardson, a former New Mexico governor who broke with Hillary Rodham Clinton in 2008 and endorsed Obama, said it was critical for the White House to "re-energize the base that got him elected: minorities, young people and independents. They need to get excited again."

Kerrey said that the enthusiasm was "quite high," but added, "It's not going to be the naive enthusiasm of four years ago."

"A real threat"

Re-election campaigns are seldom as inspiring as election campaigns and Obama, from the moment he moved into the White House, had to grapple with the expectations he had raised during the campaign.

David Axelrod, a senior Obama adviser, said the 2008 convention was "a moment in time that you really can't compare anything to."

"I don't want to sound Pollyannaish, but we are a unified party, much more than the Republican Party, that had to have sideshow tents to accommodate all their factions," he said.

Richardson said there had been a "political maturing in the party that has eliminated a lot of this carping and this whining."

"I think there is a realization that we have a real threat here," he said.