Representatives Ken Calvert and Young Kim will vie for the same district, underscoring the dilemma faced by state Republicans as their territory dwindles after the passage of Proposition 50.

Five districts held by Republicans become potential Democratic pickups, but some remain highly competitive. And a number of incumbent lawmakers have easier pathways to re-election.

Proponents say voters are motivated by stopping President Trump. Republicans have already begun pointing fingers over lackluster fund-raising and coordination.