Presidential elections during the last quarter of the 19th century tended to be exceedingly close, so close that a single campaign mistake could easily cost an election. In 1884, for instance, Republican candidate James G. Blaine ran into problems when one of his supporters called the Democrats the party of "rum, Romanism and rebellion." Blaine's failure to immediately disassociate himself from the obviously anti-Catholic slur cost him thousands of Irish Catholic votes, the popular vote in New York, and, as it turned out, the presidency. ... Recently, South Dakota Sen. Tim Johnson had a Lott moment of his own. At...