In the Florida Senate race, you have Charlie Crist and Kendrick Meek splitting the left-of-center vote, allowing Marco Rubio to run away with the race with just 44% support. Democrats should be desperate to get Meek out of the race, right? Not necessarily. Markos Moulitsas explains the logic:

The truth is that Democrats aren’t just happy for Meek to stay in the race, they are actively boosting what is pretty much a hopeless candidacy. Why? Because Meek’s presence on the ballot helps Democrats in the governor’s race. ...
holding the governorship would be a huge assist to Obama’s reelection bid in 2012, as keeping Florida blue will be a top White House priority. In addition, Florida’s governor has a veto over the state’s congressional redistricting in 2012. While a ballot initiative aims to strip that power from the partisan Legislature into more impartial hands, holding the governorship will be critical if that effort fails. 
And that’s where Meek comes in. Thirteen percent of Florida voters are — like Meek — African-American, and right now polls show him with 71 percent support in that community. That vote will be critical to Democrat Alex Sink’s chances in the virtually deadlocked gubernatorial race. In a campaign where every vote will prove critical, Democrats can’t count on Crist delivering new votes to other Democratic candidates. Abandoning Meek for Crist would almost surely depress African-American turnout and cost Democrats elsewhere on the ballot.