WASHINGTON — For the past half-century, there has been a reliable political dynamic at every presidential inauguration. Someone on the platform — usually the president or the vice president — had already emerged as the party's likely candidate for an election that was still four years away.

Which is what made the scene outside the Capitol here on Thursday morning so unusual. For the first time since the inaugural of 1953, the president and vice president were at the stated end of their elective political careers.

At the same moment, Democrats, thoroughly out of power in Congress, are adrift in their search for a leader, much less a candidate for 2008, after a debilitating loss in November.

"If you go back and look, 2008 will be the first election in modern times when there is no heir apparent on either side," said Matthew Dowd, who was a senior adviser to President Bush's presidential campaign. "It's amazing. It's a happenstance of history."

Gov. Bill Richardson of New Mexico, a Democrat who has been mentioned as a potential candidate for 2008, said: "You have a totally wide-open field with no leading candidate and no 800-pound gorillas on each side. You're seeing generation changes in both parties, and you're seeing totally new faces emerge."

Even before Bush officially began his second term, that happenstance had produced an atmosphere of uncertainty that shadowed Thursday's festivities and is poised to influence politics and policy for the president and the Democrats.

Bush and Vice President Dick Cheney, who has forsworn another run in politics, had their morning. But they were surrounded by Democrats and Republicans who represent the face of a very crowded presidential race and are already maneuvering for advantage in what scholars described as the most wide-open election since before Dwight Eisenhower entered the race in 1952.

Bush could glance around him, as he waited to take the oath, and see not only the Republican Sens. Bill Frist of Tennessee and John McCain of Arizona but also the Democratic Sens. Hillary Rodham Clinton of New York and John Kerry of Massachusetts, just four of the many people who have signaled interest in running in 2008. Rudolph W. Giuliani fended off questions on CNN about whether he could see himself walking down Pennsylvania Avenue in 2009, while Kerry sent out an e-mail message on the eve of the inauguration to supporters announcing his opposition to the nomination of Condoleezza Rice as secretary of state.