I'm not sure I'd go quite as far as Atrios on this one. I think he, and lots of other professional or at least very involved members of "THE LEFT" have a pretty good handle on the politics of their preferred policies, but I don't think most do.
In general, I think that most people believe that their preferred policies are popular. And even those who are most certainly wrong can find/create polls that will support their belief on any particular issue--just think about US deficit/debt for a microsecond.
On the other hand there are also lots of ideas championed by people who know their pet idea is horribly unpopular, but are very confident that it is the best idea (the gas tax is a great example of this).
I have to admit that I've lost sight of what the pundits are there for. It seems that they should be there to explain policies and actions to people who don't understand, but that in practice all they do is use their position to tell people what they believe and that others should too! This isn't really all that different from what the army of bloggers does. To the extent that there are people who [seem to] get both the policy and the politics and who then try to convey that to others, I'd guess that fractions of political bloggers and pundits are about the same, but by sheer numbers, bloggers win.
...also, too, the more successful bloggers, like Atrios, are generally the better ones out there, so I guess the addendum is: compared to blogs that actually get read, pundits suck.