It was well known in the early 70's that O'Niell voted for Hubert Humphrey in '68. This was in response to attempts at the time to lable him a Nixon stooge.
The demonrats cannot stand the truth. Even when it comes from another liberal. Though, I doubt that voting for Humphrey would qualify him as a liberal by today's standards.
O'Neill has voted for both Republicans and DemocRATS. He has said that his wife and kids are more conservative than him. That works in his favor. It added to David Schippers credibility that he had twice voted for Clinton.