President Obama is going to address the nation tonight about health care. Despite all the hype, it’s not likely to change much in terms of public opinion.

Over the last few years, I’ve frequently cited political science research showing that presidential speeches usually fail to change public opinion on domestic policy issues. From Bill Clinton’s failed push for health care reform in 1993 to George W. Bush’s ill-fated effort to add private accounts to Social Security in 2005, past administrations have repeatedly overestimated their ability to change public opinion. The reason is simple — the president’s message is typically offset by that of the opposition. In the aggregate, the effects tend to cancel out and the numbers don’t move.

(John Sides has an especially nice post on this point at the Monkey Cage in which he debunks the claim that Clinton’s approval surged after the 1993 speech.)

What’s so striking is that reporters and politicos alike still don’t understand this point. Why? One explanation is that people tend to conflate domestic policy with foreign policy, where the president has more freedom of action, the public has less information, and the opposition is often more deferential to the president (and is thus less likely to offset his message). In addition, some cases like the Reagan and Bush 43 tax cuts in which the president raised the salience of a relatively popular issue (but didn’t change public opinion) are often misinterpreted. Finally, there’s a tendency to explain away past failures via post hoc narratives of failed presidential leadership, bad communication strategy, etc. However, it’s not likely that any kind of leadership or PR tactic can overcome an offsetting opposition message and change public opinion on a controversial domestic policy issue under normal political circumstances.

Update 9/9 4:39 PM: As I said above, politicians just don’t understand — Obama’s own allies in the party are raising expectations to an unrealistic level:

[Senate Majority Leader Harry] Reid said he thinks Obama’s speech will clarify the debate.

“I have every belief that when he finishes his speech tomorrow, the American people will be able to put aside some of the ridiculous falsehoods that have been perpetrated these past few weeks,” Reid, D-Nevada, said Tuesday.

A House Democrat said Obama’s specifics could be a game-changer in answering Americans’ anger and concern over health care reform, displayed in sometimes violent and rowdy town halls over the summer.

“The president is clearly not running away from this battle but rather confronting the challenges we’ve encountered these last few weeks head-on,” Democratic Rep. Charles Rangel of New York said. “He’s pulling out all the stops, and this level of involvement from the president could well be a game-changer.”

Rangel said the speech could be a great way to turn public opinion on health care around.

A recent CNN/Opinion Research Corp. poll showed that Americans are evenly split over whether to support or oppose Obama’s health care plan.

Six in 10 younger Americans support the plan; six in 10 senior citizens oppose it.