Rasmussen polled of public opinion about Clint Eastwood.

Because the poll was conducted before and after his unusual performance at the convention, you get some insight:

While Republican officials were uncertain about Eastwood, 78% of GOP voters have a favorable opinion of him. A new Rasmussen Reports national telephone survey finds that Democrats are evenly divided: 42% favorable and 44% unfavorable. Among those not affiliated with either major party, 58% view Eastwood favorably, and only 21% have a negative view....

The survey was conducted on the night Eastwood gave his speech and the night after.  Interviews conducted on the first night were mostly completed before the actor’s prime time speech. Comparing the two nights shows virtually no change in perceptions of Eastwood among Republicans voters.

However, on the first night, 49% of Democrats had a favorable opinion of Eastwood. That fell to 34% on the second night after news coverage of the fact that Eastwood was making jokes at President Obama’s expense. The number of Democrats with an unfavorable opinion of Eastwood grew from 31% to 56%.  Put it together and the Democrats’ net assessment of Eastwood went from a +18 before the speech to a minus 22 after. 

Among unaffiliated voters, positive reviews of Eastwood showed little change over the two nights. However, negative views of the star grew from 12% before the speech to 29% after.

So, what does this mean? Why did only the Democrats' opinion shift — and shift so much? I would conclude that Eastwood's performance was successful. Democrats feel bad about that. It's also possible that some Democrats were unaware of Eastwood's political preference, and having found out, they don't like him anymore. The quality of the performance — in that light — doesn't matter.