The turning point?

On the 4th of July, the New York Times had a quiz about American history.  One of the questions was "What question, posed to Senator Joseph McCarthy by the Army lawyer Joseph Welch in 1954, is often cited as the unraveling point of McCarthy’s anti-Communist campaign?"  The correct answer was "have you no sense of decency?"  They add, "McCarthy’s national popularity disappeared overnight, and he died three years later" and link to the official website of the Senate, which says almost the same thing:  "Overnight, McCarthy's immense national popularity evaporated." 

The Gallup Poll had a number of questions about whether people had a favorable or unfavorable opinion of McCarthy.  This figure shows the percent favorable minus percent unfavorable (there were two different versions of the question, which I indicate by different colored dots):

The third vertical line is the date of the Welch/McCarthy exchange.  There's not much sign that it made any difference.  Moreover, McCarthy's popularity was not "immense" before it happened--in the last survey before it happened (late May), 33% were favorable and 43% unfavorable. 

Just looking at the numbers, it seems that there is one event that might have caused a lasting drop in McCarthy's popularity--a critical TV documentary by Edward R. Murrow that ran on March 9.  There seems to have been a downward trend even before that, but there was a large decline between March 2 (+10) and March 24 (-8 and -15) and support for McCarthy never bounced back. 

Why is the idea that "have you no sense of decency" was decisive so popular (I have heard it before)?  It's a satisfying story--people saw the exchange and recognized McCarthy for what he was.  If you say that public opinion was influenced by elites, like Murrow or the Senators who decided to have hearings on accusations against McCarthy, that raises questions.  What if Senate Republicans had stuck together behind McCarthy?  What if Murrow hadn't decided to do the program, or if the network executives had refused to let him run it?  Should journalists express a point of view rather than just report the facts?  It's more comforting to believe that people spontaneously saw the truth than to think about those kinds of things.

[Data from the Roper Center for Public Opinion Research]