This is a really good look at why polls under count Democratic votes.
In 2010, the polls underestimated the Democrats in every competitive Senate race by an average of 3.1 percentage points, based on data from The Huffington Post’s Pollster model. In 2012, pre-election polls underestimated President Obama in nine of the 10 battleground states by an average of 2 percentage points.
Over the summer Isis – the Islamic State of Iraq and Syria – defeated the Iraqi army, the Syrian army, the Syrian rebels and the Iraqi Kurdish peshmerga; it established a state stretching from Baghdad to Aleppo and from Syria’s northern border to the deserts of Iraq in the south.
[....] Now, more than half of likely 18-29 year-old voters want a Republican-led Congress, according to a new poll from the Harvard University Institute of Politics. It’s a marked shift for the youngest and largest generation of voters, who have supported Democrats reliably since 2004 [....]