SACRAMENTO, Calif. — Support for former eBay chief executive Meg Whitman in the Republican race for California governor has fallen by 23 percentage points over the past two months as her rival, state Insurance Commissioner Steve Poizner, has gone on the attack, according to a poll released Wednesday. Whitman now leads Poizner 38 percent to 29 percent among Republicans who said they were likely to vote in the June 8 primary, the survey by the Public Policy Institute of California found. Whitman had 61 percent support in the same poll taken in March compared to 11 percent for Poizner. Since then, Poizner has begun spending millions of dollars on television and radio advertising, much of it directed at Whitman and criticizing her position on illegal immigration, her ties to Goldman Sachs and her poor voting record. Both candidates have funneled millions into the race.