Interesting, and of course utterly unthinkable in today's political economy. Perhaps a more interesting question is, what ONE thing could be changed since, say, 1920, to bring about a world in 2016 or 2020 where these would be viable candidates? Coolidge/Hoover do more for civil rights? Eisenhower spares the lives of Julius and Ethel Rosenberg? Stevenson wins in 1952 and the Rosenbergs are executed anyway? Mainstream GOP embraces Roe v. Wade? Teddy Kennedy remains pro-life, as he was until 1972? Ditto for the Rev. Jesse Jackson? The possibilities are endless.
I suggest the most likely is Ford/Brooke defeats McCarthy/Symington. In 1980, Gov. Sam Yorty/Sen. Howell Heflin defeats VP Brooke/Speaker Millicent Fenwick. From there, the reversal of OTL accelerates as Democrats become right-populists and Republicans become left-libertarians.