So what the hell is going on in Colorado?

[IceSpear]

Where does this whole "Democrats close well" thing come from? Buck? He was just as much of a loon as Angle, and you didn't see Heller or Sandoval suddenly lose out of nowhere, nor does that stigma exist AT ALL in Nevada. It is about the candidate, and Gardner is a fantastic candidate. Is he up 6? No. Is it conceivable that he is up about 2? Sure.

While Heller didn't lose, he did come a lot closer to losing than the polls said he would. As for Buck, while he is the most notable case of Colorado Democrats overperforming the polls (because it actually changed the eventual victor), it's a notable trend regardless. Here's the data:

2004 RCP: Bush +5.2
2004 final margin: Bush +4.7
Bias: R+0.5

2004 RCP: Salazar +4.5
2004 final margin: Salazar +3.9
Bias: D+0.6

2006 RCP: Ritter +18.7
Final margin: Ritter +16.8
Bias: D+1.9

2008 RCP: Obama +5.5
Final margin: Obama +9.0
Bias: R+3.5

2008 RCP: Udall +12.5
Final margin: Udall +10.3
Bias: D+2.2

2010 RCP: Buck +3.0
Final margin: Bennet +0.9
Bias: R+3.9

2010 RCP: Hickenlooper +4.0
Final margin: Hickenlooper +14.6
Bias: R+10.4

2012 RCP: Obama +1.5
Final margin: Obama +5.4
Bias: R+3.9

Average Republican bias: 1.9 points

As you can see, the trend didn't really seem to exist in 2004/2006, so it's possible that the recent trend is just a coincidence. On the other hand, if you're only looking at the last two elections, the bias would be an enormous 6.1 points in favor of the Republican. Combine that with the fact that vote by mail would be neutral for Republicans at best and a major boost to Democrats at worst, Democrats definitely have good reason to be cautiously optimistic in Colorado.

[IceSpear]

And here's Nevada, since you mentioned that:

2004 RCP: Bush +6.3
Final margin: Bush +2.6
Bias: R+3.7

2006 RCP: Gibbons +4.0
Final margin:Gibbons +4.0
Bias: None

2008 RCP: Obama +6.5
Final margin: Obama +12.5
Bias: R+6.0

2010 RCP: Angle +2.7
Final margin: Reid +5.7
Bias: R+8.4

2010 RCP: Sandoval +16.7
Final margin: Sandoval +11.8
Bias: R+4.9

2012 RCP: Obama +2.8
Final margin: Obama +6.7
Bias: R+3.9

2012 RCP: Heller +4.0
Final margin: Heller +1.2
Bias: R+2.8

Average Republican bias = 4.2 points.

No comment needed here. Only in one race has there not been a significant Republican bias in Nevada polls in the past decade. If ever there was a state where "unskewing" is needed, Nevada is it.

[IceSpear]

You make some good points regarding Colorado, but even if the polling theory doesn't hold up when you exclude certain firms, the fact remains that even trustworthy and reliable firms have had a slight Republican skew in Colorado in the past two elections. Additionally, there's the vote by mail factor. I already did an analysis of this in another thread, but in the other two states with vote by mail, one had little to no effect on the polling averages (Oregon) while the other consistently underestimated Democrats (Washington). This inserts even more uncertainty into the equation, especially since this is Colorado's first vote by mail election. The "catch" is that both of these uncertain factors are in the Democrats' favor. Either the polling is going to not have a Republican skew or it will, meaning the effect will be either neutral or favoring Democrats. Same for vote by mail, there's really no scenario in which one could imagine this benefitting Republicans, meaning the only possibilities are that it will be a neutral factor or will favor Democrats. There's the possibility that the polling average could favor Democrats, but it seems very slim considering the history of the state.

As for Nevada, Miles beat me to it. I'm not sure about the other races, but the only reason Shelley Berkley isn't a senator right now is because a bunch of Obama voters couldn't stomach her and voted for NOTA or a fringe independent candidate.

[IceSpear]

These were the numbers I got.

It seems the polling in Washington is pretty consistently more Republican leaning than the final results.

2004 RCP: Kerry +4.5
Final margin: Kerry +7.2
Bias: R+2.7

2006 RCP: Cantwell +13.3
Final margin: Cantwell +17.0
Bias: R+3.7

2008 RCP: Obama +13.0
Final margin: Obama +17.2
Bias: R+4.2

2008 RCP: Gregoire +3.4
Final margin: Gregoire +6.4
Bias: R+3.0

2010 RCP: Murray +0.3
Final margin: Murray +3.8
Bias: R+3.5

2012 RCP: Obama +10.5
Final margin: Obama +14.9
Bias: R+4.4

2012 RCP: Cantwell +20.4
Final margin: Cantwell +20.9
Bias: R+0.5

2012 RCP: Inslee +1.0
Final margin: Inslee +3.0
Bias: R+2.0

Average Republican bias: 3 points

[IceSpear]

These were the numbers I got.

It seems the polling in Washington is pretty consistently more Republican leaning than the final results.

2004 RCP: Kerry +4.5
Final margin: Kerry +7.2
Bias: R+2.7

2006 RCP: Cantwell +13.3
Final margin: Cantwell +17.0
Bias: R+3.7

2008 RCP: Obama +13.0
Final margin: Obama +17.2
Bias: R+4.2

2008 RCP: Gregoire +3.4
Final margin: Gregoire +6.4
Bias: R+3.0

2010 RCP: Murray +0.3
Final margin: Murray +3.8
Bias: R+3.5

2012 RCP: Obama +10.5
Final margin: Obama +14.9
Bias: R+4.4

2012 RCP: Cantwell +20.4
Final margin: Cantwell +20.9
Bias: R+0.5

2012 RCP: Inslee +1.0
Final margin: Inslee +3.0
Bias: R+2.0

Average Republican bias: 3 points

Again, 2008-2012 is a consequence of keeping Rasmussen and sometimes Strategic Vision in the average while sometimes excluding the Washington Poll. True, the elections from 2004 and 2006 (excluding the gubernatorial race, which you excluded for some reason) seem to have shown a polling bias in favor of Republicans, but it would seem to undermine the argument that this is attributable to vote-by-mail if these bias has diminished in recent years.

There was no gubernatorial race in Washington in 2006. If you're referring to the one in 2004, I didn't include it because RCP didn't have an average for it. Same for the Senate race in 2004. Doing my own makeshift average, the gubernatorial race would've had a 1.8 point Dem bias and the Senate race would've had a 3.7 point Republican bias, so little impact on the overall margin anyway. You can exclude certain polls from the averages to reduce the bias as you have, but all of the aggregation and projection sites will not be doing so. True, some like Silver will attempt to correct for this house effect, but most people will be taking them at face value.