Pew Research has released a poll looking at how public support for war in Libya has changed since the president’s big speech outlining the mission’s objectives. The survey shows Republican opposition to the decision growing, while Democrats are becoming more supportive:

While the overall balance of opinion about the Libyan air strikes has remained stable, the issue is eliciting a decidedly partisan reaction for the first time. Over just the past week, Republican opposition to the air strikes has grown substantially -- 41% now say it was the wrong decision, up from 29% a week ago.

By contrast, Democratic support for the airstrikes has increased — 59% now say it was the right decision, up from 49% last week. As a result, while Republicans were at least as supportive of the decision to take military action in Libya a week ago, there is now a substantial divide along partisan lines.

This probably isn’t surprising to political scientists, but it illustrates the problem with the popular fallacy that by simply using the “bully pulpit,” the president can dramatically shift public opinion in his favor.

While it might be easy to attribute the numbers to the president losing his speechifying mojo, these numbers are pretty typical of how the public reacts to presidential speeches. As Brendan Nyhan has explained, even Ronald Reagan, supposedly the “Great Communicator,” found that his big speeches were more “likely to lower his approval and generate more public and congressional opposition than support.”

That said, support for the operation has grown slightly, from 47 percent to 50 percent, even as public opinion seems to be sorting itself back into partisan categories. A majority, though, still says the operation has no clear goal. This suggests that the President’s effort to use the bully pulpit to convince the public of the clarity of his mission just made public reaction more partisan and reinforced the sense that the mission is muddled

Speeches aren’t the only way to show leadership, but they’re the easiest ones for political writers to point to and they make for compelling (or at least easy to write up) political drama, so we tend to believe they have more of an impact than they really do.