You've confirmed what I've said. They sample/weight to meet demographics but not party.
Political party is one of the demographics that is used in the weighting.
No, it isn't.
They do the sampling of 400 people polled or 1000 people polled or whatever and then weight the results according to some pre-determined criteria. In the case of most of the media polls, they weight their sampling according to the demographic breakdown, including political party, that was the case in the 2008 election.
Also, not true. Polls are certainly weighed by demographic information, but party identification is fluid, not constant.
People who were black in 2008 are likely to still be black, so race is used as a demographic for weighing. Similar with age, and socio-economic indicators. But party identification is fluid.
So if fewer blacks or fewer Catholics or fewer Democrats or more or fewer of any particular demographic votes in the 2012 election, their polling data will moist likley be off by that much.
As I pointed out above, people's party identification changes, whereas race and religion generally don't. Which is why polls DON'T weigh by party identification.