Barack Obama will be rolling out new job proposals in a major early-September speech. Zachary Goldfarb and Peter Wallsten have the still-murky details about the specific policies the president will embrace. The question on the policy end is whether Congress will adopt the presidents suggestions (not likely, depending on the specifics) and if they were adopted, what difference they would make.

Another key question is this: Is Obama’s speech likely to have a significant impact on public opinion? Political science tells us that presidential speeches don’t have much of an impact. Does that make it a bad idea? No, not really.

Presidential speeches can make modest contributions to agenda-setting, so if there’s a chance that a big speech can change the direction of inside-the-Beltway conversations, then it’s worth a try. In particular, one key question is whether a big speech partly focused on jobs can quiet criticism of the president from high profile liberal commentators.

Many of them are not inclined to give Obama’s rhetoric the benefit of the doubt. As Jamelle Bouie mentioned recently, Obama gives full-throated rhetorical defenses of liberal values all the time, but critics don’t notice. But a major prime-time address is different — it will get people’s attention. Whether voters watch or not, the people who write liberal blogs and host left-leaning talk shows are likely to be temporarily mollified by the speech — and as a result may direct their fire at Republicans for a little while.

So, sure, the president should go ahead and give a high-profile speech. But what Obama really needs is economic growth, and what he needs for that is to find some mix of policies that would actually work, that he supports, and that he can get implemented despite a hostile House of Representatives.