A term you often hear through around politics is RINO. (Republican in Name Only). A classic example of this is New Jersey Gov. Chris Christie. Many tea party Republicans do not like him and think that he's too moderate and too liberal. Many think that he's not a real Republican, as he has many moderate policies. There has never really been a DINO, but Jim Matheson may be our nation's first.
Jim Matheson is only voting with the Republicans to get reelected. He is in a very conservative district in very conservative state, and in order to get reelected in 2014 keep getting reelected after that, he will need to side with the Republicans on some issues, even if that goes against what he believes in. Democrats may have thought about replacing him, but it is probably not a good idea. While he does not vote with them a lot, a Republican, especially a conservative Republican from Utah would not vote with them at all. With the House of Representatives currently narrowly controlled by the Republicans, and the Democrats try to take it over in 2014, or greatly narrow the margin, they need every vote that they can get and running someone other than Matheson would not be a good idea.
Link to Matheson's Website
Congressman Matheson's Congressional Portrait