Ideology and Party Identification: Voting Age Population vs. Registered Voters

 In my Honors Texas State and Local Government course on Thursday, we were talking about political ideology. One student asked if the chart that I presented on Ideology of Texans from 2010 to 2020 was of all Texans or just registered voters. The data were from the Texas Politics Project's poll and were from registered voters. Is there a considerable difference in political ideology and partisan identification between Texas' voting-age population (VAP) and Texas' registered voters? 

There is a considerable difference. Using data from the Texas Lyceum Poll from January 2021, which considers a random sample of Texas' VAP, and the Texas Politics Project Poll from February 2021, here are the results:

The percentage of moderates increases substantially; the percentage of liberals decreases by 11 percent; and the percentage of conservatives decreases by 9 percent. The results could have been expected as those Texans who are not registered to vote are less civically engaged and less likely to express a political ideology.

The partisan identification exhibits the same differences: