While all eyes have been turned toward the dramatic nominating contests in both political parties, the fundamentals that will shape the general-election contest have been shifting toward the Democrats.

Candidates seeking to succeed a two-term incumbent of their own party face an uphill battle. All other things being equal, political scientists find, such candidates can expect to fall short of the incumbents’ re-election vote share by at least 4 percentage points. Applied to the 2012 results, this metric would yield a 47% share...