I worked for several years as a Software Engineer for big Polling company. Take it from me -- polls can be systematically manipulated to produce the desired result. The question's wording, the sample demographics and the weighting that is applied to the results are all "art" not "science". Many polling practices are based in theoretical assumptions.
But the reputable polling companies do not produce many of these "public opinion" polls or partner with the MSM organizations. Public opinion polls may produce headlines and send our Dummies hearts a-thumpin', but these polls are not big revenue producers.
There is BIG money to be made in the market research arena.
Companies are willing to pay dearly for the opinions of the consumers of their goods and services. One example that was extremely lucrative is the pharmeceutical industry.
The drug producers need the polling companies to guage the effacacy and side-effect occurence of their products.
However, in my experience the market-research polls are conducted more scientifically than the public-opinion polls. The influence of the political agenda of the polls sponsors does not affect the methodolgy. The polling companies would lose credibility if the results were deemed unreliable. A polling company's reputation is their only asset.
i.e. The polling planchette.