Barack Obama took the oath of office as a United States senator on January 4, 2005, and promptly began running for president. Very quickly, he began using the peculiar kind of celebrity that comes with being a senator to introduce himself to Washington, to a new generation of political power brokers, and, more broadly, to the American people. ''He was running for president even as he was still getting lost in the Capitol's corridors,'' the New York Times noted.

Obama pointedly bypassed the Democratic Party power structure, in defiance of the norms of behavior for a junior senator. Obama did not worry about getting better committee assignments, and he did not defer to more senior members when trying to diagnose his party's problems or in offering prescriptions to fix them. By the fall of 2006, to the surprise of even the most dedicated admirers of his vast political gifts, he was admitting publicly that he was thinking about the White House.

''With an eye on his next goal, Obama treated the Senate as a bridge to be crossed--a place to learn the conventions of Washington, win powerful friends and shape what advisers referred to as his 'political brand,''' the Washington Post reported. ''Despite meager legislative accomplishments, Obama built a reputation among many Democrats as a hard worker, a reformer, an eager learner, a smart politician.'' He came to see the Senate not as a place to do things, but a place to be someone.

Four years, two weeks, and a day after that Senate oath, Barack Obama was standing on the West Front of the Capitol being sworn in as president. The brevity of his tenure in the Senate speaks to Obama's exquisite grasp [of] how the nation's political landscape and its institutions had changed over time. More precisely, his triumph flowed from a deft understanding of the incongruous role the Senate plays in our politics, our governance, and our national mythology. Maybe more than any American politician in the last hundred years, Obama saw the Senate for what it had become: not so much the great anchor of prudent self-government that the framers intended it to be but a platform, a vehicle, a point of leverage, and, for him, a launch pad to greater glory.

Still, Obama was not the first senator to have imagined the Senate in that way. It is an old joke in Washington that more than half the senators see a future president when they look in the mirror each morning. And since 1960, the last time a sitting senator won the presidency, fifty-two others have tried. Indeed, one of the ways the Senate is least representative of the nation is how thickly it is populated with aspiring presidents. . . .

The problem, of course, is that a presidential run from the Senate hardly ever succeeds. History and habit weigh heavily against it. Only [twice] before Obama--Warren Harding in 1921 and John Kennedy in 1961--have sitting senators become president. And here's why, in the words of Tom Daschle, who came within minutes of announcing a presidential bid in 2004, before deciding against it: ''Two things develop the more time you spend here: One is a mind-set that we did it this way before, we should do it this way again, and I think that's a real burden. More importantly--and Hillary and McCain are the perfect examples of this--the longer you are here, you take on enemies. And these enemies don't forget.'' But it is a measure of the kind of ambition that the Senate attracts in the first place that of the 44 men who have served as president, 16 of them had previously served in the Senate, not to mention the many others who kept trying--Henry Clay ran for president four times and lost each time. . . .