When Joe Lieberman was re-elected to the Senate in 2006, as an "Independent Democrat," he chose to sit with the Democratic caucus in the Senate, resulting in Harry Reid (D-NV) being elected, counting the vote of Bernie Sanders (I-VT), as Democratic Majority Leader. Alas, in a 51-49 split institution, the effect has been that the senator from Connecticut holds a sword over the heads of Democrats. This has meant, in the words of an eminent expert on the Senate, "his fundraisers are well attended" by supporters of both parties.
Even though Lieberman was awarded the chairmanship of the Governmental Affairs Committee, he has voted consistently with the Bush White House on matters relating to the war in Iraq; he has constantly and righteously made the case for America staying the course in Iraq; he has posed repeatedly and prominently for photos with his Republican allies in the Senate; he has said he might vote for the Republican presidential candidate in 2008; and he has, in so many words, hinted that he would consider changing which party controls the Senate by crossing the aisle and joining the Republican caucus.
In which case, there would be a 50-50 split; and Vice President Cheney (as president of the Senate) could break the tie, on a motion to re-organize, in favor of Mitch McConnell (R-KY) becoming the interim leader of the world's greatest deliberative body. The Constitution states: "The Vice President...shall have no vote, unless they be equally divided." According to Floyd M. Riddick's "Senate Procedure," the Vice President may not only vote on legislative matters, but also in the case of the election of an officer of the Senate. This precedent was established in the 19th century when a chaplain of the Senate was being chosen.
Therefore, in the case of a 50-50 tie vote to determine which party's leader becomes Majority Leader, Vice President Dick Cheney is in a position to throw the Senate back into Republican hands.
With the steady recovery of the health of Tim Johnson (D-SD), the loss of party control by the Democrats has become less likely--barring a Lieberman switch. And a quick survey of the third of the Senate seats at stake in 2008 indicates the distinct possibility of the Democrats picking up a few seats, taking into account retirements, which might help to deter a Lieberman defection.
It may be that, all along, Sen. Reid has had an ace-in-the-hole, i.e. a Republican senator--speculatively, someone like Chuck Hagel (R-NB) or Gordon Smith (R-OR) or Olympia Snowe (R-ME)--prepared to cross the aisle and counter-balance a Lieberman double-cross that would upset the balance of power in Washington.
The death of Sen. Thomas, however--if it is assumed that Wyoming Gov. Dave Freudenthal, a Democrat, is allowed some time in appointing a successor from one of three finalists chosen by the state Republican party--is a potentially important event in temporarily marginalizing the implicit Lieberman threat. According to Peggy Nighswonger, Wyoming's elections director, under state law the governor has five days to appoint one of the party's three nominees once he receives the names. That person will serve until the next general election in 2008.
But, today, the Senate contains 49 Democrats, 1 Independent, 48 Republicans, and Joe Lieberman.