There is a school of thought that national polls don’t matter when it comes to presidential nominations, and that upsets in the early contests of Iowa and New Hampshire can change everything overnight. Nonsense. Iowa and New Hampshire can give an underdog candidate enough of a boost to go on campaigning, but the national media generally write that candidate off as soon as he or she loses a contest. (Case in point: John McCain in 2000, who became George W. Bush’s main opponent by winning New Hampshire, then was tagged as a hopeless cause after losing a single primary, in South Carolina.) National front-runners, by contrast, can lose several early contests and prevail in the long run. (Case in point: Walter Mondale in 1984, who lost New Hampshire, Florida, and several other states to Gary Hart before his rival got tripped up in Illinois.)