The two biggest reasons why Sam Brownback's endorsement of John McCain matters today: 1.) Brownback has a bona fide organization in Iowa, and McCain appears to be competing there. As Jonathan Martin points out, it's not clear whether that's the right move--some would argue he should ignore Iowa and go "all-in" in New Hampshire. But, given that it appears to be the strategy, the Brownback endorsement certainly helps. 2.) As I noted yesterday, white evangelicals have been moving toward McCain lately at a surprisingly strong clip. This endorsement ratifies that and, to some extent, may accelerate it. (Though we shouldn't get carried away with Brownback's influence among evangelicals nationally. For that matter, Brownback himself isn't even an evangelical any more, though he has a lot of evangelical fans.)
--Noam Scheiber