When Americans go to vote tomorrow, a new breed of activist will be on guard, monitoring polling stations for everything from voting-machine glitches to long lines to registration snafus. Energized by disputed results in 2000 and 2004, they have left jobs as music conductors, real-estate agents and software engineers to form groups that expect to turn out thousands of volunteers who don't trust the country's ability to count its votes and have decided to do something about it. "This is going to be the most heavily watched election in history," predicts Marybeth Kuznik, who founded a group called VotePA after...