I remember in 2004 Bush lost by 10 points but in 2008 Obama won it by 30 points which was a 20 point swing in favor of the democrats.
I'd say probably the 1990s, California was barely won in 1988 by Bush Sr and the gap only widened through the 1990s. Bush 2004 was a high point for the GOP.
Bush Jr. only losing the state by 10 only looks like an accomplishment when you look at the elections afterwards. It's actually the only time a Republican President won reelection without carrying the state. Also, he lost the state by 10 when he was winning the national popular vote by 3. In 1996, Dole lost the state by 13 when he was losing the national popular vote by 8. Clearly, it was trending Dem.