After California’s then-Attorney General Kamala D. Harris announced felony pimping charges last year against the two owners of Backpage.com — a classified-ad website that is a hub for sex trafficking and prostitution, one of the men cut a $10,000 check to House Minority Leader Nancy Pelosi’s super PAC. Mrs. Pelosi’s political action committee, House Majority PAC, has resisted giving the money back, and an aide to Mrs. Pelosi said the California Democrat knows nothing about the contribution. The uproar over sexual harassment that began with the Harvey Weinstein scandal has intensified the scrutiny of political contributions linked to Backpage, which...