Mr. Obama had visited Washington only a handful of times before taking office, and he was fresh enough to its ways that he bubbled over about his first trip on Air Force One in June 2005. He fretted about getting lost on his first trip to the White House, for a reception the day he was sworn in, and later marveled that there were flat-screen televisions in the Lincoln Bedroom.

But he remained ambivalent about the city and its institutions. Unlike many senators with young children, he did not move his family to the capital. He rarely spent more than three nights in Washington  aides would reserve tickets on several flights to make sure he got home to Chicago after the final Senate vote of the week.

Mr. Obama found the Hill a difficult place to fit in, and it was not always clear that he wanted to. He was 43 when he arrived, younger than most of his colleagues  whose average age was 60  and even many senior staff members. Unlike senators who come up through the House, he did not have an existing network of friends, and while some members of Congress bunk with others, he lived by himself in one of the nondescript new boxes along Massachusetts Avenue. On the nights he was in town, he typically went alone to a Chinatown athletic club  not the Senate gym  or attended events on the Hill.

And for all his efforts to play down his celebrity, Mr. Obama was exceptional, and it was hard for him or anyone to ignore the aura and sense of history around him. He was only the third black senator elected since Reconstruction. His memoir was on The New York Times’s best-seller list for 54 weeks. And Washington society was eager to embrace him  a Capitol Hill newspaper ranked him as No. 2 on its list of most beautiful people.

Etching a Path

Mr. Obama was also pulling in big money. He created a political action committee, the Hopefund, to increase his visibility and help other Democrats. It raised $1.8 million the first year.

In the Senate, meanwhile, he was discovering the realities of being a senator  that not every bill is perfect (or perfectly unacceptable) and that most votes required balancing the good and bad. Mr. Obama wanted to vote to confirm John G. Roberts Jr. for the Supreme Court, for example  he thought the president deserved latitude when it came to appointments  but Mr. Rouse advised against it, pointing out that Mr. Obama would be reminded of the vote every time the court made a conservative ruling that he found objectionable.

Mr. Obama took few bold stands and diverted little from the liberal orthodoxy he had embraced in the Illinois Senate. His voting record in his first year in Washington, according to the annual rankings by National Journal, was more liberal than 82.5 percent of the Senate (compared with, for example, Mrs. Clinton’s 79.8 percent that year).