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The wave that has carried Barack Obama to eight consecutive primary and caucus victories since Super Tuesday is now threatening Hillary Rodham Clinton's defenses at the pinnacle of the Democratic Party.

In 2007, Clinton dominated elite endorsements -- a dynamic crystallized by her early lead among the party's unpledged superdelegates. But within the Democrats' leadership class -- that is, elected officials and constituency group leaders -- the trend is sharply toward Obama. Even more strikingly, he is drawing that support from ideologically diverse figures who champion divergent, sometimes antithetical, visions of how Democrats can prosper. At the party's apex, Obama is now executing a left-right pincer movement against Clinton that demonstrates both the breadth of his appeal and the potential conflict at the core of that appeal.

In January, from the New Hampshire primary through South Carolina's, Obama unveiled a succession of endorsements from such centrist red-state Democrats as Sens. Tim Johnson of South Dakota, Claire McCaskill of Missouri, and Ben Nelson of Nebraska, and Arizona Gov. Janet Napolitano. Kansas Gov. Kathleen Sebelius soon joined them.

Since South Carolina, Obama has solidified his support on the left. First came the passing of the Camelot torch from Sen. Edward Kennedy of Massachusetts on January 28. Then came rapid-fire endorsements from The Nation (the Left's flagship magazine), the left-leaning California branch of the Service Employees International Union, and MoveOn.org, the often-militant 3.2 million-member online liberal group. Other prominent liberals, from feminist leader Kate Michelman to firebrand Rep. David Obey of Wisconsin, have followed.

Opening a third front, Obama has accumulated testimonials from what might be called the Legacy Caucus: Democrats Caroline Kennedy and Maria Shriver, and Republican Susan Eisenhower, granddaughter of Dwight Eisenhower.

After Iowa, Clinton also attracted some prominent support, including from liberal Rep. Maxine Waters of California and centrist Pennsylvania Gov. Ed Rendell. But the biggest names from both left and center have gravitated toward Obama. One reason for this tilt is that many Democratic leaders of all ideological stripes consider Obama a stronger, less polarizing general election candidate. But he is also drawing such broad support because different elements of his party are focusing on different elements of his message.

Liberals are attracted to Obama's views on foreign policy, where he stands to Clinton's left; centrists like his domestic policy, where he has challenged liberal conventions more than Clinton has on issues such as merit pay for teachers. The wider divide is over Obama's governing strategy. Most attractive to moderates is Obama's potential as a mediator -- his promise to "reach across party lines ... and to bring people together," as Sebelius says. Most attractive to liberals is Obama's potential as a mobilizer -- his ability to excite and activate voters. "Our members really believe to make change, you don't just need a president, you need a movement," says Eli Pariser, MoveOn's executive director.

These contrasting motivations could easily collide if Obama wins the presidency. In theory, his red-state supporters like his mobilizing effect (in their endorsements, Sebelius and McCaskill cited Obama's impact on their children.) In practice, an energized grassroots progressive movement might push a President Obama toward liberal positions that red-staters could not easily adopt.

Conversely, many liberals consider Obama's promise to bridge the partisan divide naive or even misguided. Pariser warns that if Obama concedes too much to Republicans in his search for unity, "I'm not sure the movement that he is building will let him do it." Put another way, Pariser expects Obama the mobilizer to constrain Obama the mediator.

So far, Obama has skillfully courted these twin factions without being defined by either. As president, he might also balance them -- using popular mobilization as leverage for negotiating bipartisan agreements. The more immediate question is whether Obama's gains among elected officials will overcome Clinton's lead among superdelegates, which is keeping her close despite the edge he has opened among pledged delegates.

Some Clintonites fear a superdelegate stampede toward Obama if he caps his February dominance with a March 4 upset in Ohio or Texas, states whose relatively downscale demography favors Clinton. With Ted Kennedy and Ben Nelson already locking arms, the Clinton team is right to worry.