Barack Obama's optimistic campaign rhetoric has crashed headlong into the stark reality of governing. In office two months, he has backpedaled on an array of issues, gingerly shifting positions as circumstances dictate while ducking for political cover to avoid undercutting his credibility and authority. That's happened on the Iraq troop withdrawal timeline, on lobbyists in his administration and on money for lawmakers' pet projects.

It's the same delicate dance each of his predecessors faced in moving from candidate to president, only to find he couldn't stick exactly by his word. Each was hamstrung by his responsibility to the entire nation and to individual constituencies, changes in the foreign and domestic landscapes, and the trappings of the federal government and Washington itself.

"Candidates make promises and presidents break promises, and that's a very predictable pattern," said Julian Zelizer, a Princeton University historian....

"It's far easier to campaign in a purist kind of way than to govern," said Thomas Cronin, a presidential scholar at Colorado College."Reality shapes what presidents do" — and how presidents adjust to it shapes the public's perception.

Franklin D. Roosevelt ran for office promising to balance the budget. But he reversed course when he took over a country in depression and doled out a spending prescription to revive the economy. He made other shifts as well.