Looks to me like this plunge, which lasted through three quarters of Bush's term, is seeking a bottom, sort of like that which was reached one quarter of the way into Clinton's term. Henninger crows that Obama's numbers have continued to trend downward in the wake of HCR and the treaty signature with Russia. All in good time, my dear, all in good time...you're thinking in weeks rather than years.
Obama looks to me on course to lead a long climb in this headline number akin to that which transpired under Clinton. Of course, whether the turn may come before or after the November elections is the political question of the hour.
UPDATE: after posting, I take a little reading break, and wouldn't you know, Jonathan Chait made the same point three hours ago -- and natch did a better job, adding a chart