How many people would Republicans have to turn away from the polls in which key states to toss the election to Romney? I can foresee a situation in which Obama is the clear winner in the popular vote, but Romney takes the Electoral College.
But educating voters that may be at risk will help minimalize the effect. Volunteer to help people get their photo IDs, birth certificates, whatever they'll need. If you don't have time to volunteer, just getting the word out in your community will prepare folks for what to expect.