WASHINGTON -- Senators Hillary Rodham Clinton of New York and Barack Obama of Illinois are already rewriting the script of the 2008 Democratic presidential campaign, driving potential Democratic rivals to the sidelines.
Trading on their star power, capacity to raise tens of millions of dollars with relative ease, and ability to dominate news media attention, the two senators are casting a huge shadow over all others who may run.
What once shaped up as a sizable field of Democratic candidates is now shrinking. Senator Evan Bayh, Democrat of Indiana, announced on Dec. 16 that he would not seek the Democratic nomination, a surprising decision that came just days after he had witnessed firsthand Obama's drawing power in New Hampshire.
As Bayh drew small crowds on his seventh trip to the Granite State earlier this month, Obama enjoyed sold-out audiences and saturation coverage on his first trip there.
Bayh became the third Democrat to quit the race before Clinton or Obama had taken formal steps to enter. Former Virginia Governor Mark R. Warner and Senator Russell Feingold of Wisconsin abandoned their bids after lengthy periods of exploration.
All chose not to run for their own reasons, but Obama's sudden emergence creates a significant obstacle to those hoping to become the alternative to Clinton, the front-runner in the Democratic nomination contest.
"Simply put, it's the Obama factor," said Democratic pollster Peter D. Hart.
"Candidates who used to do careful exploration with the hope that they could catch fire in Iowa and New Hampshire and move from there recognize that there's no oxygen left out there for their candidacies," Hart said.
Republicans have their own celebrity candidates in Senator John McCain of Arizona and former New York mayor Rudolph W. Giuliani, but they cast a far smaller shadow on their rivals.
Those so far sidelined in the GOP race -- Senators Bill Frist of Tennessee and George Allen of Virginia -- have landed there through their own mistakes, not the looming presence of the two early poll leaders.
Dominating candidates are not new to presidential campaigns, nor is it uncommon for some politicians to explore a candidacy but never run.
In 1992, many prominent Democrats chose not to run, fearful that President George H.W. Bush could not be defeated. In 2000, George W. Bush was the clear front-runner for his party's nomination, but the winnowing did not begin until late summer of 1999 -- nine months later than is happening this time.
Even though neither has announced for president, Clinton and Obama have demonstrated the advantages of celebrity status in a world of constant cable news and expanding Internet communities.
That culture serves to reinforce the advantages of celebrity, repeatedly focusing attention on the celebrities rather than paying close attention to the doggedness of dark horses -- at least until serious campaigning begins.
A poll released yesterday shows Obama running about even with Clinton among likely voters in the New Hampshire primary.
Among participants in the Concord Monitor poll, 22 percent said they would vote for Clinton if the primary was held now, and 21 percent said Obama. That put them slightly ahead of former Senator John Edwards of North Carolina, with 16 percent.
At this point, Clinton and Obama are eclipsing a group of Democratic heavyweights that includes the party's presidential and vice presidential nominees in 2004, Senator John F. Kerry of Massachusetts and Edwards.
They also are leading other senators and governors, including senators Joseph Biden of Delaware and Christopher Dodd of Connecticut and governors Bill Richardson of New Mexico and Tom Vilsack of Iowa.
Edwards plans to launch his campaign this week. The others are still weighing when or whether to jump in.
Bayh saw his opening to run after the Democratic Party's losses in 2004. He presented himself as a Democrat who could win Republican states, such as his home state of Indiana.
But after the Democrats' victories last month, Bayh's advisers found that his potential in red states was less appealing to Democratic activists looking toward 2008.
Vilsack concluded there is still room for a dark horse. He also is hoping to ignite his candidacy on his home turf -- in the Iowa caucuses.
William Mayer, a political science professor at Northeastern University who has written extensively about the nominating process, said Clinton and Obama both appeal to Democrats on a symbolic level.
"The Democrats would dearly love to elect the first woman or black president," he said. "Given that, it's going to be tough to run an insurgent campaign against these people."