I doubt that I’d thought about William Proxmire in years, until Mark brought him up recently. And the McCain connection then seemed obvious. Now, as is always comforting, obviousness is validated by research:

Keith T. Poole, a professor of political science at the University of California, San Diego who studies the ideological voting patterns of members of Congress, said Mr. McCain had the most unpredictable voting record of any current member of the Senate. “Since the end of the Second World War, only William Proxmire of Wisconsin has been more variable,” he said, noting that since Mr. McCain entered the Senate as a middle-of-the-road Republican two decades ago, his votes have oscillated between conservative and liberal.