At his blog, Brendan Nyhan takes a look at whether Obama’s big health care speech of a month ago shifted public opinion on health care. His conclusion:

There was a small bounce in support for health care reform after the speech, but part of the effect dissipated. Meanwhile, estimated opposition to reform, which dipped in the wake of the speech, quickly rebounded toward previous levels and is now greater than it was before the speech.

This is not unexpected, as presidential speeches rarely have much effect on how voters view an issue. Nyhan’s comment that “there’s a misperception among journalists that the president can easily move public opinion” is a bit overbroad—at least some of the reporting and commentary at the time did acknowledge the limitations of public appeals—but it’s a useful corrective against the general tendency to treat political theater as major news events.

Of course, there is another scenario in which speech might turn out to be consequential—if it galvanizes dispirited members of Congress, rather than the public at large. One media narrative circulating after Obama’s speech was that it had done just that. Part of the appeal of this theory, of course, is that it can’t really be measured—and thus can’t be disproven—by polling.

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Greg Marx is an associate editor at CJR. Follow him on Twitter @gregamarx.