Senator Mark Udall at a campaign event on October 25, 2014, in Thornton, Colorado. Photograph by Marc Piscotty / Getty

Over the past week, pundits and analysts alike have become distinctly bearish about the reëlection prospects of Mark Udall, the Democratic senator from Colorado. The polls seem to echo their enthusiasm for Udall’s Republican challenger, Cory Gardner, but just barely. According to the Princeton Election Consortium, which I founded, Udall trails Gardner by a median of 1.5 percentage points. Interpreting this margin as indicative of inevitable defeat neglects the checkered history of midterm polling accuracy. Leading analysts, including those from The Upshot, may be in for a few surprises on Election Night.

By historical standards, Gardner’s lead over Udall is unusually close. Between 2004 and 2012, only thirteen Senate races had margins of fewer than three percentage points in the week before the election. Of these, four were won by the trailing candidate. In a fifth race, between Betty Castor and Mel Martínez in Florida in 2004, the polls were tied in the home stretch. Martínez, the Republican candidate, won the election, but Democrats won nine out of the thirteen close races over all.

As people line up at the polls, seven Senate races show median polling margins of fewer than three percentage points. In addition to Colorado, the close races are to be found in Alaska, Georgia, Iowa, Kansas, New Hampshire, and North Carolina. Given the rate at which trailing candidates pull off victories, we should not be surprised to see two or three upsets once the elections are all settled. (Note that Alaska may take weeks to finish counting, and that the Georgia contest might go to a January runoff election.)

The chart below shows the tremendous amount of uncertainty in 2014 compared with past elections. Horizontal bars show the range of possible seats controlled by Republicans based on polls completed in the week before the election. Any race with a median polling margin of fewer than three percentage points is scored as potentially being won by either major party. The arrowheads show the eventual election outcome.

Currently, if Republicans won every race in which they led, they would end up with fifty-one seats, the exact number they need to take control of the chamber. (They would need only fifty if Greg Orman, who is running in Kansas as an Independent, won his race and decided to caucus with them, but most believe Orman will side with the Democrats.) Considering the uncertainty of the close races, the Republicans’ likely haul ranges from forty-eight to fifty-four. Most people, myself included, think that the G.O.P. has a good shot at taking control, but a sixty-per-cent chance of victory—which is the Princeton Election Consortium’s current prediction—does not by any means spell certainty.

A major challenge to prognosticators comes from the fact that, through no fault of their own, they may have an over-all bias. The polling industry is under substantial pressure, as voters become harder to reach by landline, to continue producing good data. Three researchers—David Rothschild, of Microsoft Research, and Sharad Goel and Houshmand Shirani-Mehr, of Stanford—analyzed polling results from 2012 and found that pollster error was substantially larger than the reported statistical margin of error. They also found that Democratic candidates tended to outperform surveys by several percentage points on Election Day.

Nate Cohn, of the Times, has reviewed potential pollster error and suggests that so-called coverage error—defined as the failure to contact each likely voter with equal probability—and other problems lead to an undersampling of Democratic-leaning voters. I have found that, in the close Senate races of 2012, Democrats outperformed polls by a median of 2.7 percentage points, and that, in 2010, Democrats outperformed polls by 3.7 percentage points. Rothschild and his co-authors suggest that defects in polling arising from coverage error are unlikely to have been fully corrected this year, especially since Internet-based polling is still in its early stages of development. They suggest that Democrats may well outperform poll aggregates.

This isn't to say that Democrats are guaranteed to outperform polls again. At FiveThirtyEight, Nate Silver has reviewed polling data going back to 1990 and found that the over-all bias can go in either direction. My examination of his numbers found that the bias he noted runs much larger in midterm years than in Presidential-election years. Indeed, the median value of the bias in midterm years is 2.9 percentage points—favoring Democrats in some years, Republicans in others.

Since people’s poll responses don’t align precisely with the choices that they make on Election Day, it may be that midterm voters are particularly movable in their opinions in the home stretch—or perhaps pollsters simply face more technical challenges in estimating who is likely to vote. In other words, all poll aggregators, including the Princeton Election Consortium, could well be off in their estimates. We won’t know in which direction until after the election.

All this could be good news for Udall. If Rothschild and his colleagues are right, the ability of pollsters to reach Colorado’s hard-to-survey populations may matter quite a bit. In 2010, the Democratic candidate, Michael Bennet, outperformed surveys by 2.7 percentage points, right in the range of Democratic outperformance that year. It was enough to win him victory over the Republican candidate, Ken Buck, and to confound me and other pollsters. In a time when polling analysts have proliferated with alarming speed, perhaps the only thing we can all bank on is uncertainty.

Read more analysis and commentary at our 2014 midterms hub.