By Keith Barry The Chevrolet Volt has joined baseball and apple pie as an unassailable, non-partisan symbol of American can-do and know-how. General Motors won't start selling the plug-in hybrid for another 18 months (at least), but that hasn't kept it from becoming the most important political accessory since the flag lapel pin. The two leading presidential candidates have practically tripped over each other meeting with GM CEO Rick Wagoner, who stopped bailing water long enough to tell the candidates what they can do to help his sinking company build cars that won't destroy the planet. Both Sen. Barack Obama and Sen. John McCain have hailed the Volt as the very definition of American ingenuity, but McCain seems especially enamored by it. “The eyes of the world are now on the Volt. It's the future of America and the world, ” McCain told a crowd of 500 GM autoworkers Friday at the GM Technical Center in Warren, Mich. The way McCain sees it, American-made fuel efficient cars will save both the environment and the economy, and he promises to make sure they're built if he's elected president. McCain's promised a $5,000 tax credit to people who buy zero-emissions cars, but since there aren't a whole lot of those on the road right now he's offering a graduated tax credit that would apply to lower-emissions cars like - you guessed it - the Chevy Volt. But his biggest initiative is a $300 million prize to whoever creates a first batter ...