That itself would be something of a change.
Only twice before have voters sent a sitting senator from one end of Pennsylvania Avenue to the other: Massachusetts Democrat John F. Kennedy in 1960 and Ohio Republican Warren G. Harding 40 years earlier. And never have two sitting senators competed for the presidency as the Democratic and Republican nominees - a possibility if John McCain can ride his growing lead into the Republican nomination.
While a president McCain, Hillary Rodham Clinton or Barack Obama wouldn't need to spend time getting to know Washington, that's no guarantee of peaceful White House relations with Congress or an easy transition.
Sure, an ex-senator would take hard-won alliances and friendships with him or her to the White House, points out Julian E. Zelizer, congressional historian at Princeton. But so too would he or she take built-up animosities.
McCain and Clinton in particular should not expect a honeymoon.
"The personal back-and-forth would start right away," Zelizer said. "I think senators would be very comfortable testing these people in the White House."
Traditionally, voters pick presidents who were governors, executives adept at budgeting and comfortable issuing commands. This year the only remaining presidential candidates who have held that job are Republicans Mitt Romney of Massachusetts and Mike Huckabee of Arkansas. They have based much of their appeal on not being Washington insiders but have fallen far behind McCain for the GOP nomination.
Several senators said dealing with a president with senatorial experience would be a refreshing change after eight years of pitched battles with President Bush, a former governor.
"It would be different and I think it would be better frankly," said Sen. Sam Brownback, R-Kan., himself a former presidential candidate. "We've gotten in this situation where it's just really hard to make government work on big things."
A former senator would "know the rules of the Senate, they know they need 60 votes and they know they need to put together coalitions to get things done," said Sen. Dianne Feinstein, D-Calif.
While governors can foster an image of action, legislators are most often seen deliberating - a skill required to survive in the Senate. The place runs by consent more than rules. And that requires the regular practice of rituals that include complex dealmaking, maintenance of fairly intimate relationships, procedural gymnastics and sometimes outright kissing-up.
Despite their Capitol Hill experience, Kennedy and Harding had difficulty establishing authority over their former colleagues.
Kennedy in particular was young and faced a Congress full of lawmakers who scored better in their own districts than he did during the 1960 election, according to Senate associate historian Don Ritchie. He lost major legislative battles in the first two years.
"The old bulls, the people who chaired the committees, didn't necessarily think they owed Kennedy anything," Ritchie said. But over two years leading up to the midterm elections, Kennedy "showed his toughness" - particularly during the Cuban Missile Crisis - that won Democrats a larger majority in Congress.
"Kennedy sort of established the coattails in 1962 that he didn't have in '60," Ritchie said.
Either of the two Democrats could be in for a period of political hazing as president, especially if their former Senate colleagues suspect they used the chamber merely as a step to the White House, some experts said. Clinton is in her second term; Obama, only in his first term.
McCain, meanwhile, has a carefully maintained image of being a maverick.
Asked Monday whether he would follow the example of Senate Republican leader Robert Dole, who left the Senate when he got the Republican nomination, McCain said, "Honestly, I have not thought about it." McCain wasn't even sure it was the right move for Dole. "Whether it was right or wrong, I'll leave to the historians. But he said he couldn't do his job as Republican leader and run for president effectively at the same time."
An effective role model for an ex-senator in the White House would be Lyndon Baines Johnson in the first few years of his administration, experts suggested. But Johnson had been Senate Majority Leader for years, an experience none of this year's aspirants can match.
The sharp-tongued Texan took with him his intimate knowledge of the Senate, its members, their flaws and their interests when he became Kennedy's vice president. As president after Kennedy's assassination in 1963, Johnson used this knowledge to prod his old colleagues into enacting policies Kennedy had been advocating. He won the 1964 election with the widest popular vote margin in history and promptly used those 15 million votes as a mandate to win the enactment of Medicare.
But even Johnson saw his relations with Congress deteriorate over the bloody mire of Vietnam, for which no legislative skills could compensate. Johnson abruptly withdrew his candidacy in the 1968 election.
