Mitt Romney remains favored to win next Tuesday’s Wisconsin primary, which is arguably Rick Santorum’s last chance to prove he’s competitive. A new NBC-Marist poll shows Romney with 40 percent support, Rick Santorum with 33 percent, Ron Paul with 11 percent and Newt Gingrich with just 8 percent.

It appears there just aren’t enough very conservative evangelical voters in Wisconsin to give Rick Santorum the advantage:

The Wisconsin race follows a familiar pattern: Romney holds the advantage over Santorum among liberal and moderate Republicans (43 percent to 24 percent), conservatives (42 percent to 33 percent), non-Tea Party supporters (42 percent to 31 percent), and those who earn $75,000 or more annually (47 percent to 32 percent).

Meanwhile, Santorum leads among very conservative primary voters (42 percent to 33 percent), strong Tea Party supporters (40 percent to 32 percent), and evangelical Christians (40 percent to 29 percent).

So far in all the GOP contests where there has been exit polling, Romney has won in every contest where evangelical voters have accounted for less than 50 percent of the electorate. And he has lost in every contest where that number has been higher than 50 percent.

The evangelical percentage among likely Wisconsin GOP primary voters, according to the NBC/Marist poll: 41 percent.

The phrasing in that excerpt serves to reinforce the idea that Rick Santorum wins evangelicals generally and that Mitt Romney continues to have some kind of “evangelical problem,” but, if what we’ve learned in recent exit polling holds true for Wisconsin, that’s not quite right. My introductory sentence is more accurate: It’s not that there aren’t enough evangelicals to give Santorum an advantage. It’s that there aren’t enough very conservative evangelicals to throw the primary to the former Pennsylvania senator. Ronald Brownstein explains:

In response to a National Journal request, Langer analyzed the cumulative results of the 18 state exit polls conducted so far to more precisely track the trends in the GOP race. In the exercise, he segmented both evangelicals and non-evangelicals based on whether they considered themselves very or somewhat conservative, moderate or liberal, and then analyzed the support in each group for Romney, Santorum, Gingrich and Ron Paul.

Among evangelical Christians who consider themselves very conservative, Santorum held a commanding 41 percent to 23 percent advantage over Romney, the analysis found. (Gingrich has also carried 29 percent of this group.) That’s a powerful asset for Santorum because evangelicals have cast a 53 percent majority of all votes in the exit polls so far, and fully 45 percent of them identify as very conservative.

But among every other segment of evangelicals, Romney led Santorum. Romney led Santorum by 36 percent to 29 percent among evangelicals who identify as somewhat conservative, and by 39 percent to 29 percent among evangelicals who identify as moderates. Romney also held a 10-percentage point edge among self-identified liberal evangelicals (a small group).

When I spoke to Rick Santorum early this year, he emphasized that he planned to reassemble the Reagan coalition. At the time, much of his messaging had as its target audience “the working man,” blue-collar workers who might or might not be socially conservative. Like the rest of the candidates, Santorum primarily talked about the economy, but his strong foreign and social policy stances convinced him that he was the only true three-legs-of-the-stool conservative in the race. Somewhere along the way, though, he was rebranded the social-issues-only guy. Perhaps it was the president’s contraception mandate — such a ripe invitation for a social conservative response — that did it. Whatever it was, it crippled Santorum and narrowed his slice of the GOP electorate. Whether it’s possible for him to broaden his base now is a matter of speculation, but he’ll have to if he wants to be able to make a case for himself at a contested convention, which is itself looking increasingly unlikely.