The month of April is a big one in the GOP nomination battle, with major states in the Midwest and Northeast up for grabs, and more than 300 delegates at stake.

Who has the edge in these contests?

To start, let’s recall the basic dynamic of this primary season so far. Santorum tends to do well with rural voters, voters on the lower end of the socioeconomic scale, evangelical Christians, and of course very conservative voters. In states where these sorts of voters have been plentiful (like, for instance, the Deep South states of Alabama, Louisiana, and Mississippi and caucuses in the Farm Belt), Santorum has won. But in places like Arizona, Illinois, and Massachusetts, where these voters are less numerous, he has struggled.

With that in mind, let’s take a look at Wisconsin, the next big state up for grabs. To do that let’s compare the share that these key groups made up in the 2008 Wisconsin primary (which was at the tail end of the competitive season, as John McCain was just finishing off Mike Huckabee) to their share of the other big Midwestern primaries we have seen in 2012.

Wisconsin is kind of a mixed bag for Santorum. In terms of demographics – i.e. rural living and socioeconomic status – he has a real opportunity there. Wisconsin Republicans should be more rural than any of the other states. However, in terms of ideology and especially religion, Romney appears to have the edge, as Wisconsin Republicans are more moderate in their ideology and more mainline in their religious beliefs.

Right now the RealClearPolitics average has Romney leading Santorum by 10.5 points in Wisconsin. That seems a bit on the large side to me, given the nature of the electorate, but the internals of the Rasmussen poll show that Romney is breaking into Santorum’s base groups. The latter only leads by 4 points among the “very conservative,” 7 among evangelicals, and Santorum is actually losing low income voters by about 10 points. If those numbers ultimately hold, then Romney will win comfortably; that will be the first real sign of momentum shifting we have seen to date, with Romney actually poaching core voters off his key opponents.

In the East, Santorum is going to be hard pressed to hold his own. He is not even on the ballot in the District of Columbia, and the voting composition in the key states do not favor him at all.

There are precious few Santorum voters in these states, which is good news for Romney. The latter should carry all of these easily, and he is bound to be the heavy favorite in Delaware and Rhode Island. Though neither state had an exit poll taken in 2008, they are demographically very similar to New York, Maryland, and Connecticut.

That leaves Pennsylvania. What to make of it? On the one hand, it is Santorum’s home state, so he should have an advantage there, at least in theory. But demographically it is not a homerun for him. My back-of-the-envelope calculations suggest that the GOP electorate in Pennsylvania is similar to that of Michigan in terms of ideological and socioeconomic profile, so that suggests a closer race (as well as the most recent Franklin & Marshall poll).

So to tie all of this data together, I would draw three big conclusions.

First, Romney is all but assured of expanding his delegate lead this month. We should keep a particular eye on the Empire State and its 95 delegates. It is winner take-all by congressional district, and a 10- or 15-point statewide victory by Romney should lead to an enormous delegate haul.

Second, the other two states to watch are Wisconsin and Pennsylvania. Santorum should in theory be competitive there, and wins in either of them should keep his campaign alive. Yet if he loses both of them the pressure for him to drop out will be almost insurmountable.

Third, if Santorum can get through April with his voting coalition intact, he will face an easier time of it in May. Assuming Romney does not start poaching his voters, he stands to do well in West Virginia, Nebraska, Kentucky, Arkansas, and should have at least a chance for a big win in Texas. Of course, that is a bit if. April looks to be a tough month for the former Pennsylvania senator.

Load More