I went to great lengths to debunk the over-sampling of Democrats in polling used across polling outfits. As a brief reminder, they use the prior election as their base model (7 percentage points more Democrats than Republicans) for consistency sake but not for accuracy sake. Only Rasmussen Reports aggressively surveys the public to get a sense of current party affiliation and weights his polls accordingly. This is why, despite hysterical protests from the Left, Rasmussen consistently calls the Presidential races better than his competitors. One of the commenters, greymarch, mentioned some good work by @NumbersMuncher showed the +3.1% lead for Obama in the current (September 15) Real Clear Politics average of national polls was based on polls where Democrats were being oversampled by on average +6.1%. Now we have this disparity in graphic detail:
As you can see the X-Axis is the % over-sampling either way: movement to the left is Republican over-sampling while movement to the right is Democrat over-sampling. The Y-Axis is the attendant Obama lead which loosely correlates to how greatly Democrats are over-sampled. The real take-away which I have mentioned the times I blog national polls is that many of those national polls are HORRIBLE for Obama, namely the ABC/Washington Post and CBS/New York Times polls where you have large Democrat over-samplings but rather small leads for Obama. This means if Obama doesn’t meet or beat his stellar 2008 turnout advantage he’s in for a drubbing on election day.
These over-samplings serve a few purposes but mainly drive down enthusiasm for Republicans while assisting the Obama campaign with “bandwagon” supporters who simply like being on the winning team (they’re real and they count). If pollsters in conjunction with the Obama campaign create a negative feedback loop for Republicans such that the marginal voter doesn’t show up (definitely a well documented top priority for the Obama campaign) and assist with the bandwagon voter — a small but meaningful voter in close elections — then Obama can create the perfect storm he needs to eek out a close victory following one of the worst four-year performances for any President in modern times (Carter is the only arguable comparable).
That is the what and why pollsters are doing the massive Democrat over-sampling this election cycle.
IMPORTANT UPDATE: As mentioned in the comments section, Rasmussen Premium subscribers know that Rasmussen Reports polls are weighted D +1, not not D -2 (or R+2 however you like it) like the original chart posted. @NumbersMucher has a chart reflecting this corrected information and the chart above reflects the corrected data.