Let's say the Left Wing of the Democratic party puts so much pressure on Leftward-leaning Democrats that they stop, for instance, inviting him to the weekly policy lunch, or begin hinting broadly that perhaps the Democratic Senate caucus should strip Lieberman of his committee assignments.
If this Iran thing begins to pick up steam, then it is not at all unlikely that Joe Lieberman will be made to feel so uncomfortable by the Democrats in the Senate (four of whom are running for President and all trying to be more anti-war than the next) that he finally decides enough is enough and he offers to vote to organize with the GOP.
Assuming Senator Thomas' replacement is in place and Senator Johnson is able to vote that takes it from 51 - 49 Democrats and Independents to 50-50.
If that were to happen Senate Republican Leader Mitch McConnell (R-Ky) couldn't get to a phone fast enough to get Dick Cheney up to the Hill for a new organizing vote.
Truth is, the Democrats should never have been in control of the Senate in the first place. If it weren't for George Allen (R-Va) running the worst campaign in modern American political history - eclipsing even Michael Dukakis - this whole thing would be moot.
Hillary Clinton campaigned for Lieberman's opponent last fall. So did his Connecticut colleague (and Presidential candidate) Chris Dodd. So did Ted Kennedy. So did Nancy Pelosi.
Lieberman has shown himself to be a man of principle in not switching parties in spite of his Democratic colleagues dancing on what they thought was his political grave.
But you might start watching for the signs. I suspect that before this year is out Joe Lieberman (I-Conn) will still be an independent, but he will be caucusing with the Republican majority in the US Senate.