IT seems like a century ago now, but it was only in 2005 that a National Journal poll of Beltway insiders predicted that George Allen, then a popular Virginia senator, would be the next G.O.P. nominee for president. George who? Allen is now remembered, if at all, as a punch line. But any post-mortem of the Great Republican Collapse of 2008 must circle back to the not-so-funny thing that happened on his way to the White House.