Public opinion surveyed in the weeks before the San Bernardino, Calif., attack show that those voters who believe Muslims are dangerous have especially favorable views of Donald Trump.

What is somewhat more surprising is how Republican voters have efficiently sorted themselves on this issue among the many Republican candidates running for the nomination. Even before Mr. Trump called for a “total and complete shutdown” of Muslim entry into the United States, his supporters were in favor of the move. His call is unlikely to offend his strongest supporters, and in fact may have been an appeal to the pre-existing attitudes of his core constituency.

Public opinion data on attitudes toward Muslims collected over the last several years corroborate that Trump supporters already shared his views on that issue. The data show that a majority of Republicans view Muslims unfavorably and, according to the Pew Research Center, Republicans are cooler to Muslims than to other religious groups like Jews, Catholics and Buddhists.