The similarities are striking: Both men are longtime senators, from the Northeast, and Jewish. Their speaking styles can best be described as uninspiring. Each has been vilified by members of his own party for a tendency to vote with the opposition. Their bids for the presidency went nowhere.

Both men can be quite gruff, but they are highly respected for their statesmanlike qualities. And they have shown considerable political courage throughout their careers, often casting votes that damaged their reelection chances based on the belief that they were acting in the best interests of their constituents and the nation.

The stories of Joe Lieberman and Arlen Specter are remarkably parallel, and the two senators are increasingly likely to have one more thing in common - rejection by their party's voters. But unlike his Connecticut counterpart, who emerged from the Democratic Party's banishment to return to Washington as an independent, Specter's alternate route back to Capitol Hill is strewn with obstacles that may mean the end of the line for the ultimate political survivor.

Specter's potential problems in next year's Republican primary are well-documented. The seething antipathy toward him within the conservative base of the state's Republican Party has created a toxic environment for his reelection bid. And with many of the GOP's moderates now residing in the Democratic Party, Specter faces a more conservative and hostile primary electorate than he has in the past.

Given these bleak prospects, one could not blame Specter for considering other ways of getting reelected to the Senate. One option would be a switch to the Democratic side. But this possibility seems remote for a number of reasons.

First, Specter's chances of winning a Democratic primary in Pennsylvania may be no better than his prospects on the GOP side. After all, an 80-year-old who has not been a Democrat since the Johnson administration is not a dream candidate for the party's rank and file.

And Specter's record of voting with the GOP 60 percent of the time may make him too liberal for today's Pennsylvania GOP. But it also makes him too much of a Republican to craft a message that can win the state's large Democratic electorate.

Given the limited appeal of a change of party affiliation, the possibility of a run as an independent appears to be increasingly in play for Specter. And what better way for the Senate's consummate independent thinker to finish up his career than by throwing off the shackles of partisanship and returning to the Senate as an unaffiliated representative of the people?

While such a finale sounds incredibly fitting - and quite Liebermanesque - it's not likely to happen for a few reasons. First, the electoral process in Pennsylvania makes an independent run more arduous than in Connecticut. In Pennsylvania, a candidate who loses a party primary cannot run as an independent in the general election. That blocks Specter from following the path Lieberman took in 2006, when he followed up his Democratic primary loss to Ned Lamont with a general election win as an independent.

This procedural impediment doesn't necessarily preclude Specter from breaking with the GOP and beginning his reelection bid as an independent. Like Lieberman, Specter has impeccable moderate credentials that should make him a potent force in a three-way race in a state with an enormous moderate voting block. But this strategy looks less sure on closer examination.

The conditions in which Lieberman succeeded as an independent are not present in Pennsylvania today. As a Democrat, Lieberman was a member of the dominant political party in Connecticut. The Republican Party has become something of an afterthought there, unable for years to put up competitive candidates for the Senate.

So when Lieberman entered the race as an independent, he controlled a solid share of Democratic voters and could dip into the Republican voting bloc in a fairly unfettered way. The exit polls from 2006 tell the story: Lieberman took 70 percent of the Republican voters in that race, with only 21 percent backing the Republican Party's nominee.

It's hard to imagine a scenario in which Specter could be nearly as successful in winning votes from Pennsylvania Democrats in 2010. This is a serious problem for him, because the Democrats make up the lion's share of the commonwealth's electorate.

So even if Specter can win a solid block of Republicans and a clear majority of independents, the electoral math would work against him. Only if the Democratic Party put forth a severely flawed candidate could Specter hope to piece together a coalition that would give him a plurality of Pennsylvania voters.

Specter seems to be keeping the option of running as an independent on the table, but he has said it would be a last resort. This appears to be as much a nod to electoral reality as it is to his loyalty to the Republican Party.

If Arlen Specter is to remain the ultimate political survivor, he will probably have to do so with one more run through the GOP primary gauntlet. If he can pull that off, he will go back to Washington the way he went there in the first place. Even Joe Lieberman can't say he did that.