Here's a wild card: 1976.
Ford wins and all the woes of the late 1970's get pinned on the GOP. Watergate, vietnam, "malaise," Iranian hostage crisis, all perceived as a 12-year Republican orgy of incompetence and corruption. There is never a Reagan Revolution (with all that entails) in this timeline! Ted Kennedy (or someone very much like him) wins a landslide in 1980, opening the door for a second Great Society with Democratic super-majorities in Congress.
For all we know if Dewey had won in 1948, it would have prevented the Republican resurgence with Ike, Nixon and Reagan.
At least one historian has suggested that a Dewey victory would have prevented the resurgence of the southern strategy; stopped the GOP from adopting its intense Red-baiting campaign; and led to a much more liberal Republican Party in the long term. It's interesting to imagine a United States divided, today, between an anti-elitist, know-nothing "populist" Democratic Party anchored in the South (but with support from working-class whites nationwide)... and a business-oriented "progressive" Republican Party drawing support from educated whites and minorities across the country! Basically an extension of the 19th century's voting patterns.
Imagine a 2012 Democratic platform calling for single-payer healthcare
and constitutional amendments banning abortion and same-se marriage.
