1876

"One of the most disputed and controversial presidential elections in American history," as Wikipedia puts it. Samuel Tilden won the popular vote and was leading by 19 electoral votes with 20 in dispute. Somewhat unlike the swing state model of today, the three deciding states were Louisiana, South Carolina and Florida. Rutherford B. Hayes successfully disputed one of the electoral votes and sued Tilden over the vote, ultimately emerging victorious when Tilden's Democratic party cried uncle. It marks the only time in history a candidate received 50% or more of the popular vote and still lost.