http://fivethirtyeight.com/datalab/senate-update-colorado-polling-is-probably-right-udall-is-losing/"It’s true that polling in Colorado has overestimated GOP support. The FiveThirtyEight projections in the 2010 Senate race and 2012 presidential race, mostly based on the polls, were off toward the Democrat by 2.7 and 2.9 percentage points, respectively.
But Gardner’s lead is nearing a point at which he could afford a polling error of that magnitude. He’s up by 2.4 percentage points in our latest projection. Udall would need every bit of those past errors to pull out the victory.
You’d also have to think the mistakes pollsters made in the past will happen again this year. But pollsters adjust their methodologies. Furthermore, two elections in a row doesn’t mean the polls “always” underestimate the Democratic candidate in Colorado. In Udall’s last campaign, in 2008, he was projected to win by 11.1 percentage points by FiveThirtyEight. He won by 10.3 points. In other words, the FiveThirtyEight estimate — again, based mostly on the polls — slightly overestimated Democratic strength....
... Finally, it’s possible that all-mail voting will throw off the pollsters, but there isn’t any evidence it will. Oregon has had all-mail voting since 2000, and Josh Katz of The New York Times has found it had the seventh-lowest error rate in Senate polling out of the 36 states he studied. Washington switched to all-mail voting for the 2012 election, and the polling average in the 2012 Senate race was off by 0.5 percentage points.....
... The polls and the fundamentals tell the same basic story: An unpopular incumbent is losing in a purple state against a decent candidate in a slightly Republican-leaning year. That’s a pretty believable story."
