Polling experts we talked to noted that registered voters are more likely to be Republican, which could lead Democrats on Morning Consult polls to have inflated disapproval statistics.
Cliff Zukin, Rutgers University professor of political science and public policy, brought up that the results from the surveys are brought from volunteer responses, which is not as accurate as a true scientific sampling method. Because the respondents would volunteer to answer the survey, it is not as representative of the underlying population as a completely random sampling method would be.
Houston Herald, October 26, 2018