Tonight, if a recent Pew Research Center poll is to be believed, a massive television audience will tune in to watch President Barack Obama deliver his big health care speech to a joint session of Congress. Obama likely believes he can entice many of those viewers to embrace his version of reform; the press will likely judge the speech’s success by how much it boosts approval for Obama personally, and for reform in general—in Slate, Timothy Noah entertainingly aggregates the coverage calling this a “make or break” moment. And it’s not just the mainstream media that will use the speech as a measuring stick. Nate Silver, the wunderkind behind FiveThirtyEight.com, has called tonight’s speech “The Biggest Moment of His Presidency.”

But there’s good reason to be skeptical that the speech, whatever its strengths or weaknesses, will have much effect in moving public opinion—because, as we’ve written before, the evidence that presidential speeches can ever move public opinion is surprisingly scarce. Yesterday, the political scientist John Sides made this point in an excellent post at The Monkey Cage. Sides quotes another political scientist, George Edwards, who found, in a study with a twenty-year sample:

…statistically significant changes in approval rarely follow a televised presidential address. Typically, the president’s ratings hardly move at all. Most changes are well within the margin of error—and many of them show a loss of approval.

Sides also provides some good analysis showing that Bill Clinton’s health care speech in September 1993—sometimes cited as an exception to that rule—wasn’t much of an exception after all. In fact, from mid-1993 to early 1994, Clinton’s approval rating was on a long-term incline. Depending on which polls you look at, the immediate aftermath of that speech may have been one of the few times his support didn’t rise.

Gary Langer of ABC News, meanwhile, accepts the premise that Clinton’s speech moved his ratings, but notes that the effect ebbed so quickly as to be more of a blip than anything else. And both Sides and Langer point out why a speech is unlikely to be a game-changer: even with a wide audience, most viewers will be politically attuned, high-information types who have already made up their minds.

Even if Clinton’s speech wasn’t an outlier, there’s at least some reason to think Obama’s might be. According to that same Pew Poll, 93 percent of respondents said health care reform was “important,” and 73 percent said it affected them personally. At the same time, 67 percent of people said the debate was “hard to understand.” If you were trying to create a profile of an issue on which people’s minds were open to change, this is more or less what you’d come up with—an audience that’s motivated to find out more information. Still, it’s better to stick with the rule than to assume that you’ve hit on the exception, so observers would be wise to keep expectations in check.

Although Obama has likely overestimated his own persuasive powers, he may have stumbled upon some fortuitous timing nevertheless. As several observers have pointed out in recent days, the outlook for the Democrats’ health care reform push is as sunny as it’s been in some time, for reasons that don’t seem to have much to do with the president’s public appeals. Max Baucus, whose slow-paced approach to negotiations helped hold up reform before the August recess, is now talking about the need for speed. Olympia Snowe, one of the three Republicans in Baucus’s “Gang of Six,” seems to be genuinely looking for a deal. Ben Nelson, probably the most conservative Democrat in Congress, is making noises about accepting some form of the “public option” liberals have been demanding.

These are the people who can make reform happen; at the moment, it looks as though they may be inclined to do so (though not, most likely, in the form that’s most appealing to Obama’s liberal base). Over the last few months, the president has taken a lot of grief—not all of it fair—for not finding some way to make them do his bidding. If he goes out and gives a great speech tonight, he may end up—not for all the right reasons—with the credit.

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Greg Marx is an adjunct lecturer at The Medill School and a facilitator with The OpEd Project. She served as an editorial board member, columnist, library director, and No. 2 in the features department of the Chicago Sun-Times.