A few weeks ago, in the course of arguing that the press was overemphasizing the role President Barack Obama could be expected to play in forcing health care reform through Congress, I wrote the following “to-be-sure” sentence: “The president, of course, has a role to play in trying to shift those incentives [facing members of Congress] by swaying public opinion.”

I still think the press has missed the mark with much of its criticism, but I may have been off-base with that remark. Because, while there is evidence that popular presidents—the sort that Obama was when the health care debate began—can use their popularity to move the public and members of Congress, there’s also evidence that they can’t. And recent research trying to make sense of the issue suggests that there was little reason to believe Obama could have substantially shifted public opinion on health care.

Some of that research has been conducted by Brandice Canes-Wrone, a professor at Princeton’s Woodrow Wilson School and the author of Who Leads Whom? Presidents, Policy Making, and the Mass Public. For those who’d rather not slog through poli-sci journal articles, Canes-Wrone helpfully summed up her views in this Q&A, conducted during George W. Bush’s second term:

When have presidents typically been most effective in influencing the American public?

One of the arguments I try to bring home is that presidents are rarely able to change mass opinion in a big way. Presidents always want to think that they can do this. You see this with the current president as well — he went out there with Social Security reform and thought he could shape public opinion. But the evidence, over time, is that voters are not that likely to change their opinions on policies simply because the president is promoting them, even when presidents are very popular. That’s a lesson that presidents find hard to learn.

There are a couple big caveats. On foreign policy, where citizens have less knowledge and fewer fixed opinions, presidents have greater ability to shift people’s views (not to mention more freedom to act as they see fit). And on issues that are technical or on which the public debate is new, citizens may be more inclined to defer to a popular president.

But “for the vast majority of domestic policies, voters have fairly well-formed preferences,” Canes-Wrone said in an interview Friday. And those preferences, once formed, are hard to move—especially because the president is “not going to have a monopoly on the message.” Another example she cites: when Ronald Reagan wanted to cut taxes early in his first term, he had the public on his side, and he was able to leverage that support into a victory over a hostile Congress. But when Reagan wanted to eliminate the departments of Energy and Education, and the public wasn’t with him, he wasn’t able to win them over (or, ultimately, turn his policy preferences into law).

In fact, when a popular president tries to swing people in favor of an unpopular policy, a more likely outcome is less-popular president, still-unpopular policy. So while pundits fault Obama for not making his pitch in the right way, Canes-Wrone sees the situation differently. “I actually think to some extent he hurt himself with these public appeals,” she said. “The mistake is believing that he could sway public opinion.”

Of course, this analysis presumes that Obama’s health care reforms are unpopular, or at least not popular enough. In fact, some polling has showed support for a “public option,” one of the most contentious elements of the various reform proposals, though more recent polls have found mixed opinion. (There is also an ongoing discussion about the quality of the polling.) But as Gallup pointed out a month ago, while many elements of reform have drawn support in isolation, the broad package has encountered ambivalence (and skepticism from voters that it would actually help them personally).

So if there’s little evidence that presidents can shift the public on high-profile issues, why do they continue to believe they can? It’s probably because presidents tend to be charismatic, successful and—to put it politely—confident people, used to persuading others and getting their way. They’re also surrounded by intelligent, ambitious aides who are ready to believe they’ve found the new strategy or technique that will succeed where others have failed. And why does the press play along, anticipating bold results and then carping when they fail to materialize? It may be that, for all our skepticism and readiness to find fault, we’re also hoping for some white knight to come along who will sweep away the hard, grubby, relentless grind of politics.

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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.