Oh dear. It's happening already: "Early voting has begun, and problems are already emerging at the polls. In West Virginia, voters using touchscreen machines have claimed their votes were switched from Democrat to Republican. In North Carolina, a group of McCain supporters heckled a group of mostly black supporters of Barack Obama. In Ohio, Republicans are being accused of trying to scare newly registered voters by filing lawsuits that question their eligibility."
America - leader of the democratic world - Not.
Will the Democrats have the guts this time to challenge Republican cheating and intimidation? They didn't in 2004.
Look for the gap between exit polls and the final result. Exit polls are very accurate. Correlation between the two is a key indicator of the fairness or otherwise of an election.