Poll results that try to show a change (or lack thereof) in an opinion over time are usually reported similarly to this answer:
As of mid-December 2018, 59% of Republicans thought it was 'very likely' that Trump would get Mexico to pay for the wall (question 6A), and 9% believe he has 'already accomplished' this.
As a comparison, in April of 2017, only 46% of Republicans thought it was 'very likely' (question 4), 47% in November of 2017 (question 6), and in March of 2018, it was up to 54% (question 6A).
However, one theory I've seen for these numbers is that it's (roughly) the same quantity of people who would respond "Very likely", but the number of people who identify as Republicans has gone down, and thus that group represents a larger share of Republicans.
What would be a good way to report poll results which takes changing party affiliations (or other demographic changes) into account? Should all comparative poll results just include the percentages of the population that reported being affiliated with each party? Should poll results always provide total ("all American") numbers?