The fundamentals of the presidency are very different. The approval ratings of presidents tend to decline over time and they typically have a difficult time passing important legislation after their first year or two in office (i.e. once the low-hanging fruit has been picked)...And in Obama's case, he faces a poor economy that will push his approval numbers into the 40s very soon. The one factor working in his favor are the large Democratic majorities in the House and Senate, but the Senate caucus lacks sufficient unity and discipline to push through health care and other controversial legislation in the form that he wants.
As a result, Nyhan says, "the August '08 comparison isn't particularly useful."
There are a couple of things here. First, I don't know that Nyhan is reading Kilgore fairly. I didn't read the original piece as saying people shouldn't worry because Obama always triumphs, but as a general caution that it's never a good idea to panic based on a few bad news cycles, especially with this particular politician, who makes it a point to not worry about a few bad news cycles.
Second, I think that Nyhan's general comments on presidential effectiveness are far too strong. The evidence is far to weak to support an absolute statement such as Nyhan's that "Obama's presidency -- like almost all others -- faces a downward trajectory of power and influence for the next couple of years." It's true that more major bills tend to pass in the first half of each administration (I'm looking at some old data from David Mayhew here, but I don't know that it's changed), but that's a tendency, not an iron law. Yes, Obama's approval ratings are sagging with the economy, but if the economy rebounds next year the approval ratings will no doubt follow (as they did for Reagan and Clinton). Of course, if Republicans gain in the 2010 elections it will hurt prospects for liberal legislation in the subsequent Congress, but those gains are not certain at this point, especially in the Senate, where small gains or losses matter a lot more.
He could be right. A double-dip recession, with unemployment lingering around 10% through 2011, could leave Obama will little to show for a one-term presidency beyond a mild health care bill. But it's at least equally likely that the economy snaps back nicely, Obama winds up signing a very significant health care bill into law by the end of this year, and Congress returns in 2010 to find a climate and energy compromise that is far more than any previous bill -- and that the Democrats lose seats in the House but hold tight in the Senate, leaving Obama a strong candidate for reelection against a still-flailing GOP. Or somewhere between those scenarios. We just don't know enough about the economy, about the true intentions of marginal Democrats in the Senate, or about Obama's skills to make blanket predictions.
Granted, liberals who want single-payer on health care and a stronger climate bill than Obama ran on will be disgusted with him sometimes even things go as well for him as possible, but those things weren't in the cards a month ago, six months ago, or really any time after the Iowa caucuses in January 2008. But I agree with those who predict that liberals, at least, will be quite happy with Obama if and when he signs a health care bill even if it isn't what they wanted. All in all, I think Kilgore is correct in urging people to ignore the day-to-day news cycles, and that it's much too early to start predicting limits (positive or negative) on the Obama years.