If the union succeeds, the Chattanooga plant would become the first among foreign automakers in the South to unionize. The election begins Wednesday.
That's a prospect dreaded by Republican politicians in the state, who say they worry about losing a competitive edge in drawing future business to Tennessee.
Republican U.S. Sen. Bob Corker, a former Chattanooga mayor who played a key role in bringing Volkswagen to Tennessee, has been among the most vocal critics of the union drive.
Corker on Tuesday called the vote a "quickie election" because it wasn't announced until last week and said the UAW was "stacking" the balloting in its favor.