Paul Caron has the data:
FundRace 2008 on The Huffington Post lists 635 "law professors" as having made contributions to candidates in the 2008 Presidential election totalling $623,472. Of this amount, 92.7% ($577,924) has been contributed to Democratic candidates, 7.3% ($45,548) to Republican candidates. $487,772 has been contributed to the nominees of each party — 94.7% ($461,754) to Barack Obama, 5.3% ($26,018) to John McCain.
He also has the breakout for some leading law schools.
Notice that the percentage giving Republican actually slightly exceeds the percentage giving to McCain (although probably not in a significant fashion). This is consistent with my observation that among the small number of confessed "conservative" law professors (or people in the "ideas business" generally), John McCain was not a candidate who generated a great deal of excitement. Fred Thompson, Rudy Giuliani, and Mitt Romney all seemed to be relatively more ideas-driven candidates (note that I did say "relatively") and who garnered more law prof support and excitement. And, of course, for many libertarian and conservative law profs McCain-Feingold and the view of the Constitution that it implies was basically disqualifying, at least at the time there were other apparently viable Republican options available.
Caron's report is also consistent with a hypothesis that I have heard others express, that although schools like Harvard are supposedly hiring more self-professed "conservatives" these days, they tend to be (with a small handful of notable exceptions) of a certain ilk who can make it through the meat-grinder of the appointments process--"well-behaved" conservatives who aren't going to rock the boat or otherwise challenge the prevailing culture, such as by taking public political stances or otherwise speaking out on controversial issues. I haven't done any sort of systematic analysis of this hypothesis, which presumably would require substantial first-hand knowledge.
The Harvard reference was triggered (subconsciously at least) by Steve Bainbridge's similar observations, which he makes here.