SACRAMENTO, Calif. — Nearly 20 years after California became the first state to ban the use of race and ethnicity in college admissions, a proposal to reinstate affirmative action has sparked a backlash that is forging a new divide in the state’s powerful Democratic Party and creating opportunity for conservatives.

The debate is unfolding in the nation’s most populous and most ethnically diverse state as an unrelated U.S. Supreme Court ruling upholds voters’ rights to decide whether racial considerations should factor into university selections.

The California proposal would allow voters to rescind their state’s affirmative action ban, but unexpected pushback from families of Asian descent who mobilized through Chinese-language media, staged rallies and organized letter-writing campaigns has all but killed the measure.