As the electorate closes in on selecting presidential nominees for 2008, three Senators still stand a credible shot at winning the nod. But some may find their hopes dashed, forced to reassess their futures in a chamber far less star-studded than the campaign trail they’ve called home for months.

With that in mind, questions already abound over whether Sens. Hillary Rodham Clinton (N.Y.) and Barack Obama (Ill.) — considered by many to be the Democratic Party’s new class of rock stars — would trade unsuccessful presidential ambitions this year for a lengthy career in the Senate. Or would Sen. John McCain (R-Ariz.), now 71 and facing election for a fifth term in 2010, opt to close out his political career in just over two years if his hopes of being the commander in chief aren’t realized?

“It’s a question you can’t answer yet,” said Erik Smith, former top aide to the 2004 presidential candidate former Rep. Richard Gephardt (D-Mo.). “If a candidate loses in the primary, you either come out enhanced or diminished. In large part, that dictates how you move on.”

Within weeks, if not days, the fields for the GOP and Democratic nominations will thin out, and soon thereafter, the parties should be clear on whom they will field for the presidency in November. Obama, Clinton and McCain all have legitimate shots at winning their party’s respective nominations. But not all of them can be president and all would have to set their goals elsewhere if unsuccessful.

Senators, aides and observers alike contend that both Obama and Clinton could have long and illustrious Senate careers if they choose to remain in Congress. At 46 and just partway into his first term, Obama has several decades in front of him — and could easily make a political life in the Senate, run for Illinois governor in 2010 or make a second run at the White House in four, eight or even 12 years. It’s also possible Obama’s future could hold a vice presidential nod should Clinton prevail as the 2008 Democratic presidential nominee.

Clinton’s options may be only slightly more limited. At 60 and in her second Senate term, Democratic Senators and observers suggest that Clinton may be less inclined to mount another campaign for the presidency in four or eight years, but could easily follow the model of Sen. Edward Kennedy (D-Mass.), who failed to win the White House in 1980 but has since become one of the chamber’s most powerful icons.

“They still have many years of great service ahead of them,” Sen. Ken Salazar (D-Colo.) said of Obama and Clinton. “I think that’s what will happen with whichever of the two returns.”

“Both of them have established themselves as leaders and they will be looked to for the future,” said Sen. Debbie Stabenow (D-Mich.), a declared Clinton backer. “People will come back together very quickly. They are both extremely talented and very bright and will have a tremendous amount to contribute.”

Several Democrats suggested that while Obama would likely keep his eye on a second presidential bid, Clinton perhaps would be more likely to immerse herself in the Senate — fashioning herself like Kennedy as a consummate legislator who, while principled in position, is at the heart of working out most of the institution’s landmark deals. Another possibility, several Democrats said, is for Clinton to wait for her turn as the next Senate party leader, when Majority Leader Harry Reid (Nev.) steps aside.

“I have to believe that’s been discussed, but I wouldn’t go as far as to say there’s a clear path there,” said a senior Democratic Senate aide.

Indeed, if Obama or Clinton fails to secure the Democratic nomination, it wouldn’t be the first time a Senator tried and failed to win the presidency. Kennedy is among the likes of numerous Democrats, including his home-state colleague Sen. John Kerry (D-Mass.), who narrowly lost the 2004 election to President Bush only to return to his work in the Senate.

Most recently, Sens. Joseph Biden (D-Del.) and Chris Dodd (D-Conn.) abandoned their 2008 White House hopes to reclaim their daily Congressional routines, although neither Senator was considered as credible a White House contender as Clinton or Obama.

As the senior Democratic Senate aide observed: “The Senate is littered with past presidential candidates who have gone on to successful careers.”

“There’s no reason either Hillary Clinton or Barack Obama wouldn’t want to be the best Senator for their state or the country,” the aide added. “It’s their choice.”

One prominent Democratic strategist who has worked on a series of campaigns said that regardless of who wins the party nomination, the “celebrity won’t diminish” for either Obama or Clinton, who enjoy far greater star power than any Democratic nominee in recent times. This operative noted that even if forced to exit the campaign, both Senators would have “extremely important roles to play in the other one’s election — regardless.”

“If they don’t get behind the person and help them, then that person will struggle,” the strategist said.

Sen. Ron Wyden (D-Ore.), who has seen many of his colleagues entertain presidential ambitions over his tenure, said that no matter what the outcome of the Democratic nominating process in 2008, both Clinton and Obama “are going to be major forces in American politics for years to come.” Wyden said he believes either one would return to the Senate, where they could help “drive a big part of the political debate” and influence Democrats’ agenda moving forward.

“I don’t think after presidential campaigns like that, either of them just come back and assume a career that is low key and quiet,” Wyden said. “If either came back to the Senate, my sense is they would continue to zero in on major issues that are enormously important to both our party and this country.”

In her seven years in the Senate, Clinton has developed a reputation among Democrats as a leader on issues such as foreign policy and national security, while Obama has carved out a niche as a cross-the-aisle deal-maker on issues such as ethics and lobbying reform.

“Both of them are in a position to make a huge contribution for the betterment of the country and for the Democratic Party,” Wyden said.

A second senior Democratic aide said the Conference likely would approach Clinton and Obama differently, assuming one returns to their duties full time after the Feb. 5 Super Tuesday primary. This staffer suggested that Obama likely would need little extra incentive to stay put in a chamber he’s called home since only 2004.

A post-presidential candidate Clinton, however, may be “a harder problem to solve” for Democratic Senate leaders since she is a more senior Member who may not have another chance at the White House, this source said. With that in mind, the Democratic aide said Reid may look to find a plum committee slot or even a chairmanship for Clinton to take on — a move that could help round out a long-term Senate career.

For McCain, this is his second turn at a presidential campaign, having run and lost the nomination in 2000 to then-hopeful and Gov. George W. Bush. Many believe McCain exited that race with more muscle than when he entered it, and was able to use the subsequent eight years to build a Senate career of legislative accomplishment.

“One could argue McCain sat for eight years waiting for Bush,” said one McCain ally, speaking on the condition of anonymity.

Having waited that long to run again, however, many Republicans have questioned whether McCain has another political campaign in his future — either for the presidency or even for the Senate, where he’s served for nearly a quarter-century. But McCain’s closest confidantes are all but certain the Arizona Republican would complete the final three years of his Senate term if he fails to win the White House.

“I can see him finishing out his term — I can see him really focusing on the international things he’s passionate about,” said Sen. Richard Burr (R-N.C.), a McCain backer in the presidential contest. But Burr was quick to stress that he believes McCain will not only win the Republican presidential nomination, but also the general election. “I told you a long time ago not to write his obituary,” he said.