Maybe this is mis-titled. It says that in recent elections that polling tends to under-count Democrats. But what is going on, is that polling may not be reaching certain classes of Democratic voters. Therefore, we find that Democrats has lately outperformed in actually voting the the numbers that professional pollsters generated prior to the election.
This is actually something that we have seen in a recent article in the Washington post, that finds significant voting block that would be excluded from traditional polling....
Our best guess, based upon extrapolations from the portion of the sample with a verified vote, is that 6.4 percent of non-citizens voted in 2008 and 2.2 percent of non-citizens voted in 2010.From here. And from polling of these non-eligible voters, it is found that 80% voted for Democrats. So, cheating, of course, is difficult to capture in polling particularly if it is as prevalent as this.
Additionally, since Democrats for decades have been adept at stealing elections through fake and false voting, I presume that cats, dogs and dead people, a traditional core constituency of the Democratic party is also being excluded from pre-election polling.
Why Polls Tend to Undercount Democrats - NYTimes.com