If you include non-voters

Ezra Klein - Poll: Massachusetts does not think Scott Brown should obstruct health-care reform
Who the hell cares what non-voters think? They're non-voters. They've opted out of the process, and have chosen to make their opinion about political matters irrelevant. They're too feckless and lazy to bother going into a booth and making a decision about the direction of government. You might as well poll children. Wait - Scratch that. Many children will someday open a newspaper from time to time, get up off their asses and vote.

Polling non-voters is like asking vegetarians which meat they prefer. It's like asking a nun what her favorite sexual position is. It's like asking a tourist for directions.
Via Tim Worstall