The campaign hasn’t officially kicked off yet, but supporters of a proposed sales tax increase on the ballot this November didn’t miss the chance to reach a captive audience Saturday.As riders stepped off C-Tran shuttle buses to the Clark County Fair, they were greeted by a small rally in favor of Proposition 1 — a 0.2 percentage point bump in the sales tax to preserve basic bus service. C-Tran officials have said the transit agency would have to cut service by about 35 percent if the measure fails. Those cutbacks would mean eliminating more than a dozen bus routes, plus service to special events like the fair.“This is just one of those services where people who don’t ride the bus all the time can see the benefit,” said Heather Stuart, treasurer of Keep Clark County Moving, a political action committee campaigning in favor of Proposition 1.The measure would raise C-Tran’s current 0.5 percent sales tax to 0.7 percent. That would translate to an extra 2 cents on every $10 purchase for consumers, collecting an estimated $8 million to $9 million more annually for C-Tran. The agency has said it now spends about $6 million in reserves for annual operations, and would run out of those funds in 2013 without a successful vote.Stuart said supporters plan to wait until after Tuesday’s primary election to formally kick off their November campaign. But Keep Clark County Moving has been fundraising, collecting more than $60,000 since forming early this year. It has spent almost $29,000, according to the state Public Disclosure Commission.