Marin County voters, like Californians statewide, overwhelmingly backed Proposition 50 last week, an aggressive redrawing of the state’s congressional map that could eliminate as many as five Republican-held seats and deliver a victory for Democrats hoping to wrest control of the House in next year’s midterms.

In Marin, 80.65 percent of the 107,488 ballots cast supported the measure—the second-highest approval rate in California after San Francisco—according to the county’s preliminary tally. Statewide, voters endorsed the…