There was quite a ruckus about this same issue with Chaput in 2004 -- the NY Times misquoted him, then the NY Times had to give Chaput a column so that what he really said was corrected.
Then MSNBC and CNN got involved and walked away with black eyes (both networks).
This year the convention is in Denver, and on August 12, 2008, Archbishop Chaput released his book Render Unto Caesar ... Catholic Beliefs in Political Life
There were quite a few bishops, archbishops, and Cardinals that made statements during the 2004 election.
I think Chaput is doing this to get Catholics to read his book and bring attention to Catholics not to listen to political leaders about Catholic Church teaching but rather to Church Leaders...