On Monday, the Libertarian candidate for Senate in Iowa, Doug Butzier, died in a plane crash. Butzier, an emergency room physician, was polling at only 2 percent. But Republican candidate Joni Ernst leads the Democratic candidate, Bruce Braley, by an even narrower margin. How Butzier's supporters eventually vote is a question of national significance.

Half a dozen other Senate races are equally likely to come down to the wire. That's why control of the chamber is still uncertain, even though the election is less than three weeks away. Such uncertainty is apparently belied by the fact that the major aggregators of Senate polls are pointing in the same direction, with probabilities ranging from 60 percent and up for Republican control. This includes my own site, the Princeton Election Consortium, which recently switched away from a Democratic advantage in early October.

Indeed, Republicans hold the lead in key states, but this unanimous agreement among election forecasters conceals an Achilles heel: We all rely on the same poll data. What if that data is off?

A Few Front-Runners in Close Elections Will Lose on Election Day

Josh Katz at The New York Times' The Upshot has analyzed the performance of Senate polls since 2004. He found that the predictive accuracy of polls depends on how soon the election is and the size of the front-runner's lead. For instance, if the election is three weeks away and the front-runner leads by 3 percent or less, that candidate will still lose 38 percent of the time—nearly two times out of five.

In the period Katz analyzed, only three or four Senate races in each election were decided by 3 percentage points or less. But as of Tuesday, such narrow margins existed in six races: