Like an avowedly pro-choice version of Doug Kmiec, former Massachusetts Gov. Bill Weld has gone from supporting Mitt Romney in the Republican primaries to Barack Obama in the general election. The Boston Globe item compares Weld to other Republicans jumping ship and notes Charles Krauthammer's disapproval. But former Minnesota Gov. Arne Carlson, also mentioned, was a classic liberal Republican, a Gopher State Nelson Rockefeller. His politics are probably to the left of Colin Powell's.
Weld in his prime was a somewhat different animal. He was a conservative on the issues that drove him -- taxes, spending, welfare, and crime -- but a liberal on the issues that the Boston Globe uses to disqualify Republicans from office. His liberalism on abortion and gay rights was important to social conservatives, but for him it was mainly a way to get those issues off the table so he could cut taxes, balance the budget, reform welfare, and introduce criminals to the "joys of breaking rocks." The only comparable figure I can think of is Rudy Giuliani.
Alas, toward the end of his first term in Massachusetts he began the slide to a more conventional Rockefeller Republicanism, making him the kind of guy who would endorse Obama. His New York gubernatorial bid was an unmitigated disaster, like his attempt to become Bill Clinton's ambassador to Mexico. But for a brief, shining moment, Weld was about as good as it gets in the Bay State.