In chemistry, random means that any sample from the mixture or solution would have the same composition. If the mixture is not well-mixed, or if the sample is too small, the concentration of the components would possibly differ from one sample to the next more than the experimental error would predict. In the sense that strict procedures are followed, random sampling would produce the same results every time.
Thus, sampling by Leftist polling agencies would not normally meet strict scientific requirements since they would not follow strict procedures.