What's up with Ben Nelson?

Ben Nelson, the most conservative Democrat in the U.S Senate, is so far the only member of his party to pledge support for Supreme Court Nominee Samuel Alito.

Nelson is very conservative when it comes to his voting record. He votes with the Republicans on most social issues as well as many economic ones. He is staunchly pro-life, and supports a constitutional ammendment to outlaw abortion. He opposes most gun control legislation, but most prominently, he has shown consistent support for Bush's tax cuts. He doesn't really play ball when it comes to fighting the Republicans on judicial nominations and whatnot, and was one of the first senators to come out in support of Alito's confirmation. I guess what everyone is wondering is what makes this guy a Democrat? Here is his voting record.