SAN FRANCISCO — U.S. Sen. Bernie Sanders made a fresh appeal Saturday for California voters to approve a proposed one-time tax on the state’s billionaires, as the measure’s campaign confronts declining support in polls and a large spending gap with its opponents. At a rally in San Francisco, the Vermont independent called the November ballot initiative a test watched well beyond California. “People are asking: ‘Is it possible?’” Sanders told the crowd. “And you are going to do it and show the world it is possible!” The proposal, backed by a health care union, would impose a one-time tax of 5% on California billionaires. Supporters say the revenue would help the state’s health care system absorb federal cuts. Sanders described the measure as one of the country’s most consequential ballot questions. Saturday’s appearance was his second visit to California this year to campaign for it; in February, he spoke at the initiative’s kickoff in Los Angeles. The measure has become a high-profile debate over how California should respond to reductions in federal support for health care—and whether a tax on the state’s wealthiest residents can help fill the resulting gap. Its prospects now face pressure from both weakening polling and a better-funded opposition campaign.