Most commentators think that Bush's pro-immigration stand is responsible for his recent decline in support among the Republicans who make up his base. To check the accuracy of this, I compared how Bush has fared among Republicans in two SurveyUSA polls of all 50 states. In addition to its overall results, SurveyUSA includes partisan breakdowns for each state. The first poll was in January of this year when Bush was still strong among Republicans (average of 80 percent across all 50 states) and before the immigration debate began to heat up. The second poll is the most recent one from May when his standing with Republicans had slipped significantly (down to an average of 66 percent) and the immigration debate has been in the news.
I looked to see if Bush's support among Republicans declined more among Republicans in border states or in states that have seen significant growth in their Hispanic population. In both cases, the answer is no. Overall, the average percent decline in Bush's approval among Republicans was 15 percent. In the border states of California, Arizona, New Mexico, and Texas, it fell by 18 percent. That's a difference but not a statistically significant one. In states with above average growth in their Hispanic populations, Bush's approval among Republicans declined by 15 percent--the same as his overall decline. Finally, in states with above average Hispanic populations the decline was 16 percent.
Some caveats are in order. First, these statewide polls have very small samples (usually around 600 respondents) and the subset of Republicans is even smaller, so random variation might be swamping a real effect. Also, Republican who are upset about immigration might be randomly distributed around the country and not concentrated in border states or states with large or rapidly rising Hispanic populations.
Nonetheless, this suggests that Republicans' declining support for Bush goes beyond just the issue of immigration.