http://www.powells.com/s?kw=schlesinger+vital+center&Search.x=0&Search.y=0Is probably the most classic spirited endorsement of centrism.
It's actually a very eye-opening read, because the author's definition of centrism includes American liberalism, European social democracy, the labor movement, democratic socialism, and the New Deal of FDR and the Democratic Party. Those are all things that people think of as being on the left today, thanks to the Repukes making an all out effort to reframe the language. (The book was written in 1949). The author also includes the moderate and liberal Republicans, which were a lot more common back then, as part of the centrist coalition. At the same time, he was anti-Communist and reserved some of his harshest language for those "progressives" who expressed sympathy or support for the Soviet Union, or who were willing to work in coalitions with the Communist Party. Today, that would probably apply to anyone who supports Osama bin Laden, which is a moot point here since that is not a problem among liberals (indeed, the most likely place for bin Laden sympathizers in the U.S. is among the extreme-right militia nutsos and anti-Semites.)
Centrism was promoted as the best way to fend off the dangers from the extreme left and extreme right. The extreme left was of course Marxism, Leninism, Stalinism etc., but on the extreme right, the main danger was no longer from Nazism or Fascism which had just been defeated, but from the laissez-faire capitalists, isolationists, religious fundamentalists, extreme red-scareists, etc. - the very same people who reorganized into the Birchers and Goldwaterites of the 50s and 60s, the Religious Right from the 70s on, and are now today's movement conservatives.
So, using this book as a political guideline, the Democratic Party of today is centrist and therefore legitimate and mainstream, the Republicans are extreme right and therefore a real danger to the republic. Like I said, it's an eye-opener. I no longer see centrism as something bad, but as the broad coalition of interests - including all of the liberal and progressive ones - that makes up the mainstream of politics. Today's Republicans (with a few exceptions: Specter, Chafee, Snowe, Collins, Shays) are so far to the right they are outside the realm of legitimacy. And, for that reason, I promote a "big tent" Democratic Party with room for all. The "move to the left vs move to the center" arguments miss the point: The democratic, moderate left *is* a part of the center.