Rasmussen has started running a tracking poll on head-to-heads against McCain, with numbers running back about a month; I haven’t seen people mentioning it. All the usual caveats, blah blah.

But for what it’s worth, I’ve calculated an Obama electability advantage index (TM) — (Obama minus McCain) – (Clinton minus McCain). Here’s what the Ras numbers say:

Basically, through much of last month this tracking poll showed Obama winning comfortably against McCain, but McCain winning comfortably against Clinton — which is more or less the narrative of many Obama supporters. But if you believe Rasmussen, Obama has collapsed since then, while Clinton has improved — McCain/Clinton is now an approximate tie, while McCain beats Obama by five points (hence -4%).

It’s a long way to the general. But this sort of thing should worry Obama supporters: if he is the nominee, he’ll face much worse in the way of attacks than he’s facing right now.

Add: A new WaPo poll shows both Dems beating McCain, but Obama by more. Take both Rasmussen and this poll with a pile of salt. Actually, take the whole polling enterprise well salted. As it happens, I spent early Tuesday evening with some major insider types, who had exit polls and believed that Obama had put the thing away. I took the train home, got in very late, went to bed thinking that the nomination fight was over, and opened the papers to see that it wasn’t. Nobody knows anything.