GARDEN GROVE, Calif.—Peter Nguyen has long paid attention to politics, but until last month he wasn’t registered to vote.

Since then, the 29-year-old Democrat attended his first political fundraiser, made his first political contribution and left his job as a physical therapist, he said. He recently began work as a field organizer for a national Democratic committee in Orange County’s Little Saigon, home to a crucial battleground House district.

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