Hillary Rodham Clinton

Senator Hillary Rodham Clinton has already spent far more time in the White House than the rest of the presidential field combined.

Since Mrs. Clinton began her independent political career in 2000 by running for the seat in the New York Senate being vacated by Daniel Patrick Moynihan, the assumption in political circles was that her ultimate goal is a return to Pennsylvania Avenue, this time as the spouse in charge. And for almost as long, the expectation has been that Mrs. Clinton would begin the race for the 2008 Democratic presidential nomination as the front-runner -- as she has.

John Edwards

Been there, almost done that: One distinction John Edwards holds in the crowded presidential field is being the only candidate in either party to have been on a November presidential ballot, having served as Senator John Kerry's running mate in 2004.

At that time, he was Senator John Edwards of North Carolina, the youthful, handsome and eloquent (Bill Clinton said he could "talk an owl out of a tree") millionaire former trial lawyer whose own presidential hopes had faded in New Hampshire after a strong second-place showing in the Iowa caucuses. Having declined to seek re-election to the Senate while running in 2004, he left office early the next year.

Barack Obama

Barack Obama, a freshman senator from Illinois, might be the first to admit that the excitement generated by his presidential bid has little to do with what he has already done, and much more to do with hopes about what he might do.

Hope, in fact, is deliberately at the center of Mr. Obama's political strategy -- as in the title of the introspective, highly personal best seller that he wrote leading up to his decision to run, "The Audacity of Hope."

Audacity is also a word that might easily be applied to someone pitting two years in Washington against an unusually large and experienced field of rivals. Mr. Obama's formal announcement of his candidacy was held in Springfield, Ill., the hometown of a president who had a similarly skimpy political resume.

Rudolph W. Giuliani

Most candidates for president have a carefully crafted stump speech, a talk they deliver over and over to emphasize the themes, proposals and even key words they think are at the heart of their appeal. Not Rudolph W. Giuliani.

Mr. Giuliani likes to do things his own way. So far on the campaign trail, he appears to be giving a very different talk at each stop, according to the interests of his audience and, it seems, his mood at the moment.

But there is one thing that comes up every time: Sept. 11, 2001, and Mr. Giuliani's role as a voice of calm and confidence at a time of fear and uncertainty.

John McCain

Senator John McCain of Arizona has only been defeated in one race in his political life, in a heated contest against George W. Bush for the Republican presidential nomination in 2000. Now as he runs for president again, his candidacy has become closely tied to the increasingly unpopular war that his rival began.

Mr. McCain, who was elected to the House in 1982 and the Senate in 1986, has emerged as the Republican field's strongest proponent of an aggressive approach to the Iraq war and of Mr. Bush's decision in January to deploy more soldiers, primarily in Baghdad. While other Republican candidates, including Rudolph W. Giuliani and Mitt Romney, are resisting Democratic calls for disengagement, no one else in the race is as closely associated as Mr. McCain is with the idea of adding troops. Mr. McCain's chief criticism of Mr. Bush's so-called surge is that it is likely too small to have the necessary impact.

Mitt Romney

It is not unusual for prominent Massachusetts politicians to run for president. What is unusual is for one to run by appealing to the most conservative wing of the Republican party. But little of Mitt Romney's political career has followed a conventional path.

Mr. Romney was raised in a political family. His father, George W. Romney, was a moderate Republican who ran the American Motors Corporation, served as governor of Michigan from 1963 to 1969, and was briefly a candidate for the Republican presidential nomination in 1968. His mother, Lenore Romney, Republican but more liberal than his father, later ran unsuccessfully for a Senate seat.