Once upon a time, when I was a young man just starting to pay attention to Congressional politics, we had a de facto three party system in the United States. There were Northern Democrats, Southern Democrats and Republicans. Southern Democrats parted ways with the Northern counterparts and joined Republicans on issues of national defense, civil rights and civil liberties.
This is no longer true. We still have a three party system, but, as this Daily Kos post illustrates, the three parties are now, regular Democrats, Blue Dog Democrats, and Republicans.
The Blue Dogs proudly call themselves conservative Democrats and there are 44 of them in the House out of 231 Democrats (there are 202 Republicans and 2 vacancies). Thus, there are 187 regular Democrats, 44 Blue Dogs and 202 Republicans, plus 2 vacancies. There isn't a formal Blue Dog caucus in the Senate (which has 49 Democrats, 49 Republicans, liberal independent Bernie Sanders, and moderate independent Joe Lieberman), but the number of Senators who vote like Blue Dogs is similarly a small part of the entire Democratic Party caucus.
Realignment has turned a lot of Southern Democratic seats into Republican seats, and replaced all but a handful of Rockefeller Republicans with Democrats. It has also left a more clear line between Blue Dogs and the rest of the Republican party (Markos at Kos has repeatly observed accurately that the voting record of non-Blue Dog members of the supposedly moderate Democratic Leadership Council look a lot like non-DLC Democrats). The Blue Dogs are fewer in number than either the DLC or the Southern Democrats who held the moderate reins before them. The Blue Dogs are also not nearly as distinctively Southern in character, although many are from the South. But, they still hold the swing votes on a variety of issues.