California's congressional primaries on Tuesday are a preliminary test of Democrats' best chance at countering Republican redistricting gains elsewhere this year. The party won voter approval last year to redraw the state's congressional map to create five more winnable U.S. House seats. But some Democrats now worry that they could be locked out of the November ballot in one of those districts, near San Diego. The reason: So many Democrats are running for the seat that they could split the vote and allow two Republicans to advance to the runoff. Other races elsewhere in the state are defined by the progressive-establishment split among Democrats.

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California Democrats this weekend in San Francisco have an opportunity to set the tone in a pivotal election year when their voters could deci…