In a general election, a media pollster can seriously damage their reputation by putting out a biased poll. These are the type of surveys where various organizations purchase individual questions and they don't sponsor the entire survey. Party standings from omnibus polls are usually found outside of elections. You can always find something positive to leak if you have asked enough questions. I have never heard of a legitimate research company that has fabricated polling results.
Campaigns sometimes try to release their canvass results under the guise of research.
Again, it is the context in which subsequent questions are asked. Parties, in most cases, want an accurate reading of how well or poorly they are doing. That is why neutral or even-handed questions should appear before the voting intention questions. The political outlook of the client influences the poll for the same reason - clients won't let pollsters ask all the questions that could give the public a fair chance to voice its true opinion. Sampling and response rate, more as shoddy research standards, are ways that poll results can be inadvertently tilted.