Fundraising, advertising and grassroots efforts are underway to persuade voters to approve a November ballot measure that would give Bay Area transit agencies a financial cushion and prevent major service cuts. Voters are being asked to approve a 14-year regional transportation sales tax of one cent in San Francisco and a half-cent in Alameda, Contra Costa , San Mateo and Santa Clara counties. The tax would generate about $980 million annually to help fund BART, Muni, AC Transit, Caltrain, SF…