Most statewide California candidates blow off the Central Valley. There are more votes and media — and donors, of course — on the coast. But not this year. The Central Valley is up for grabs for Senate candidates vying to replace Sen. Dianne Feinstein. The top three Democrats — who represent coastal districts in Los Angeles, Orange County and the Bay Area and aren’t well known in almond country — made a beeline last Saturday from a major union endorsement interview in Los Angeles to a $25-a-head fundraiser hosted by Rep. Josh Harder along the Stockton waterfront. … But while they’re all showing up to campaign in the Valley, none has crafted a position on a crucial aspect of the issue in America’s breadbasket that could give them an advantage over their rivals: water.  Specifically, the Delta tunnel, the proposed 45-mile conveyance tunnel through the Sacramento-San Joaquin River Delta.