Romney's advantage in the new survey rests on the fact that he's running better among less ideological and more secular voters than any of his rivals is polling among the Tea Party supporters and evangelical Christians still largely dubious of him. In the survey, Romney attracts 33 percent of Iowa Republicans who don't identify with the Tea Party, up from 26 percent in the early December poll. Similarly, he attracts 34 percent of Republicans who don't consider themselves evangelical Christians, also up from 26 percent in December. (Paul showed solid gains with both groups too.)

Romney posted more modest improvement among Iowa voters who identify with the tea party (from 15 percent in early December to 19 percent now) or describe themselves as evangelical Christians (13 percent in early December to 16 percent now.) Romney's showing with Iowa evangelicals is actually slightly below the 19 percent of them he attracted in 2008, according to exit polls.

But this modest performance is good enough for first place, at least for now, because the more conservative elements of the party skeptical of Romney are dispersing like cats in a cornfield. In the early December Iowa poll, Gingrich attracted 40 percent of tea party supporters and 31 percent of evangelical Christians - enough to give him a solid lead in the state. But in the new survey, no candidate attracts more than 19 percent of Tea Party supporters (Romney, Santorum and Ron Paul all draw that much, followed closely by Gingrich with 17 percent). Among evangelicals, Santorum leads with a modest 22 percent; Bachmann (12 percent), Perry (13 percent), Gingrich (14 percent), Romney (16 percent) and Paul (18 percent) are all clustered relatively close behind him. That's an even more fractured picture than in the 2000 caucus, when George W. Bush led among voters who identified with the "religious right" (the way the question was phrased then) with a middling 33 percent, according to exit polls.

In New Hampshire, Romney displays much broader strength. The state has few evangelicals, but the new poll shows Romney attracting 45 percent of likely GOP voters who don't support the Tea Party and 42 percent of those who do, and leading comfortably with both groups. That reverts to the pattern of broad-based Romney support from a late October CNN/Time/ORC poll there; in the early December survey, Gingrich had surged past Romney among New Hampshire Tea Party supporters. Without a boost from an Iowa win it's hard to imagine any Republican elbowing past Romney in New Hampshire. Even if someone receives such a boost, it won't be easy: in a contested race, no Republican has won both contests since the Iowa caucus assumed its modern form in 1980.

Given Romney's consistent advantage in New Hampshire, if he wins Iowa, it could quickly become very difficult for any challenger to seriously threaten him. In the modern primary era, no one has won the GOP nomination without capturing either Iowa or New Hampshire. Indeed, every contested GOP nomination race since South Carolina moved up its primary in 1980 has followed the same pattern: one candidate has won Iowa, a second candidate has won New Hampshire, and one of those two has captured South Carolina - and the nomination. There's no guarantee that pattern will hold in 2012 - but its durability suggests that candidates who don't win either of the first two contests can face enormous challenges in convincing voters in later states to still view them as viable.