I REALLY don't think George Wallace would've become a Republican, but not for the reasons that OldiesFreak mentioned.
For one, George Wallace was a very populist politician. Not only would his pro-segregation stance be at odds with the Republican Party (actually, it would've been at odds with the majority of the Democratic Party as well,
as the numbers indicate on the vote by party on the Civil Rights Act.), but so would his economic philosophy. Republicans, while not outright laissez faire in the 1960's, were at least considerably more pro-business in rhetoric than what Wallace would've felt comfortable with. At least as a Democrat he would retain his populist visage by sticking with the "little guy's party", something that he wouldn't have as a Southern Republican (who were mostly seen as rich oilmen at the time).
