How did pollsters miss the Republican tsunami?

USA Today reports:

The wave of Republican victories caught many Democrats — who were hoping the polls this year were once again undercounting Democrats — by surprise. Several poll analysts this year had speculated that the factors causing 2012 polls to underestimate Democrats' performances would re-occur — specifically citing the possibility that not enough Hispanics were included in Colorado polling and that whites were overrepresented in polls across the South.

Both problems of underestimating Republican performance in 2014 and overestimating it in 2012 can be traced to the difficulties pollsters have in trying to identify who will vote, according to Evans Witt, CEO of Princeton Survey Research.

Assumptions.