A while ago, I claimed that Obama's health care speech to Congress was far less important than some would have it.
John Sides and Brendon Nyhan have the goods on one part of why tonight's health care speech isn't that important: presidential speeches don't move public opinion very much. The logic is well stated by Matt Jarvis in a comment at Monkey Cage: "Reception: only us political junkies will be watching. Acceptance: the junkies already have decided their positions."
That's only half of the equation, however. Even if Obama's approval ratings moved, say, five points as a result of the speech (and Sides and Nyhan point out why that isn't likely), it just wouldn't be very important. If Senators Lincoln, Conrad, Landrieu, and about a dozen of their colleagues are hesitant to support public option when the president's approval ratings are at 51% nationally, they're really not especially more likely to suddenly vote in lockstep if he moves to 56% approval. Especially since Obama at 56% nationally is apt to still be no great asset in Louisiana, Arkansas, North Dakota, Montana, and the other states represented by the non-Lieberman marginal votes. That's why Bob Reich's argument ( "The best way to give Blue Dogs cover is for the President to explain clearly and boldly why the public option is essential to health care reform") is silly. Lincoln and Landrieu don't need political cover from people who are listening to the president; everyone who voted for and supports Obama is safely in their camp at this point. Now, if Obama shot up to 90% approval, that's a different story...but of course that's not a plausible outcome.
Obama might be able to give cover to liberals who are afraid of betraying their liberal constituents if they vote for a compromise bill, because those liberal constituents like Obama, and are probably willing to accept his definition of "reform." But not Democrats from marginal states.
At any rate, however much liberals may not like it, there are a whole bunch of Democrats in the Senate who believe that their electoral futures depend on maintaining a perception back home that they aren't Barney Frank/Barack Obama/Ted Kennedy liberals. The logic of their position is almost certainly going to be that they want the bill to pass (so Democrats aren't a party of failure) but without their votes (so they can't be hung by those votes). Nothing Obama can do in a speech can help make sure that they are on board, even if he was able to move public opinion a notch or two. There are things that Obama and the Democratic leadership in Congress can do, but those are going to take negotiations and bargaining, not speeches.
Obama will likely do just fine tonight, and since I think that health care reform is in pretty good shape, it's likely that reporters who mistakenly thought prospects for reform looked bleak last month will credit the speech for turning things around. But that's not what's happening, even if Obama beats the odds and manages to move public opinion a little.