Rasmussen reports is supposedly one of the best pollster today. They use automated phone calls to poll various topics including a daily job approval rate for the President. It is undeniable that they have been very accurate over the past few years in terms of political races.
It is clear that they have a Republican bias. They ask questions that Conservatives and Republicans would want. Sort of a push poll. Something like, "Do you think it is better for the economy if you keep more of your own money or spend more on government programs?".
However, I suspect they skew their results as long as they can towards the Republican. Then, as election day comes closer, they bring things back into reality.
The benefit of this is that they build a big conservative and Republican base, while also getting to proclaim that they are accurate.
So on a normal political day when Rasmussen puts out, "Republicans have 7 point generic lead_ it has credibility because they have been very accurate at election time, which is pretty much the only objective criteria we have.
But over the course of months, they can slowly change their models back to reality and get a best estimate on election day.
That's good business for them, so beware.