For some context, San Diego City's presidential electoral history:
2016: D+ 37.40 (Clinton 65.07% - Trump 27.67% - Johnson 3.87% - Stein 1.93% - Write-ins 1.20%)
2012: D+ 24.76 (Obama 61.04% - Romney 36.28% - Johnson 1.30% - Stein 0.58% - Write-ins 0.41%)
2008: D+ 26.69 (Obama 62.23% - McCain 35.54% - Nader 0.73% - Write-ins 0.55% - Barr 0.50%)
2004: D+ 11.12 (Kerry 54.89% - Bush 43.77% - Badnarik 0.44% - Write-ins 0.31% - Cobb 0.26%)
2000: D+ 10.74 (Gore 52.90% - Bush 42.16% - Nader 3.90% - Browne 0.43% - Write-ins 0.36%)
1996: D+ 10.78 (Clinton 50.57% - Dole 39.79% - Perot 6.20% - Nader 2.01% - Browne 0.81%)
1992: D+ 11.68 (Clinton 43.53% - Bush 31.85% - Perot 23.89% - Marrou 0.49%)
1988: R+ 10.73 (Bush 54.67% - Dukakis 43.94% - Ron Paul 0.80% - Fulani 0.33% - Griffin 0.26%)
1984: R+ 20.51 (Reagan 59.61% - Mondale 39.10% - Bergland 0.61% - Richards 0.38%)
1980: R+ 23.64 (Reagan 54.96% - Carter 31.32% - Anderson 11.02% - Clark 1.53% - Commoner 0.73%)
Very interesting. Hillary Clinton clearly had the best Democratic performance ever in the City of San Diego, at least as far back as your calculations go. Trump got less than 30% of the vote there, John McCain and Mitt Romney both got in the mid-30s, and George W. Bush (the last Republican to win San Diego County), is also the last Republican to manage over 40% of the vote in San Diego City. San Diego's shift away from Republicans after 1988 (the last time it voted for that Party at the presidential level) seems to correlate with the end of the Cold War, the diminution of Southern California's defensive industries, and the acceleration of unfavorable demographic trends against that Party.
