When he unseated Republican incumbent George Allen in 2006, in one of that year’s most remarkable political upsets, Virginia’s Jim Webb tipped control of the Senate to the Democrats. An military man and writer who had never sought public office before, and who preached an edgy economic populism (especially with regard to trade policy), Webb captured the mood of the moment and secured a seat that few national observers has thought was up for grabs until just weeks before the election.

Now, Webb is quitting the Senate. And his decision to leave rather than seek a new term increases the already very real prospect that Democrats will lose the chamber in 2012.

Democrats were already on shaky ground. Their commanding sixty-seat majority of 2009 and 2010 was reduced last November to a narrow 53-47 advantage. And, in 2012, Democrats must defend all those seats they won in 2006. As a result, there will be twenty-three Democratic seats in play, as opposed to just 10 for Republicans.

In swing states such as Virginia, Democrats almost always face serious contests, and Webb would not have had an easy time of it in 2012.

But Webb, an independent-minded former Republican who always went his own way (the former Secretary of the Navy recently went to the Senate floor to praise Ronald Reagan while poking at the current crop of Republicans), was probably better positioned than any other Democrat to attract independent votes and hold the Virginia seat.

Now, the party has another open race to worry about.

Webb’s decision will increase the pressure on former Virginia Governor Tim Kaine, the chairman of the Democratic National Committee, to run for the seat that Allen and a number of other Republicans covet.

Kaine’s clearly interested in the race. And he’s  an appealing figure, with a progressive record. In 2005, I profiled him for The Nation, suggesting that he was a Democrat who had some good ideas about how to “unlock the code of the South.” If Kaine could rekindle the energy he generated in that 2005 gubernatorial race, he might well be the best contender. But that would take some serious effort. Right now, he’s burdened with an image as an insider: the Virginian most closely tied to a President Obama who polls suggest is not so popular in the state as was the candidate Obama of 2008. And there’s a strong sense around Washington that he would prefer to be the Attorney General in a second Obama administration.

So what’s the smart play? If Virginia Democrats, and their national allies are really interested in competing for this seat and others like it in 2012, they will pause and look at all their prospects. The Virginia race is going to be a hard one, and the best prospect for the Democrats might well be another outsider like Jim Webb.

Webb seemed to say as much in announcing his decision to depart at the end of one term, recalling, “Five years ago this week, on February 8, 2006, I announced my intention to run for the United States Senate. We had neither campaign funds nor a staff. We were challenged in a primary, and trailed the incumbent in the general election by more than 30 points in the polls.”

“Over the next nine months we focused relentlessly on the need to reorient our national security policy, to restore economic fairness and social justice, and to bring greater accountability in our government,” he continued. “I will always be grateful for the spirit and energy that was brought into this campaign by thousands of loyal and committed volunteers. Their enthusiasm and sheer numbers were truly the difference in that election.”

If Democrats are to have a chance of holding the Virginia seat—and a number of the others that are in play next year—their first task will be to find an economic-populist candidate who can renew that outsider, insurgent energy that Webb generated in 2006.