San Diego County was a stronghold of the Republican Party for a significant chunk of the last century, with the Republican candidate for president carrying the county in all but one election from 1948 through 2004, when George W. Bush beat John Kerry in San Diego County by a healthy margin of about 52% to 46% (Bill Clinton eked out a narrow plurality over George H.W. Bush in 1992 with Ross Perot in the mix).

But just four years after the junior Bush carried San Diego County in his reelection bid, Barack Obama won it by more than 7%. He went on to win the county by a similar margin in 2012,...