Here's the thing: without understanding the intergroup dynamics of American politics, you can't understand much of anything about Santorum. This might take a while, but hopefully someone will actually read it.
I follow Mencius Moldbug in analyzing America as containing five distinct groups; more distinctly, five castes: social groups with their own internal status systems. (There's also a sixth, which Moldbug leaves out, but they're not relevant to electoral analysis, as far as I know.) The outline of the American caste system is here, but I'll summarize it because I know NG hates reading. So, the castes are as follows: (I'm quoting directly from Moldbug for a lot of this, since there's not much more I can do to summarize the relevant parts)
1. Brahmins, who define status by "scholarly achievement, success in an intellectual profession, or position of civic responsibility". If you're reading this now, you're probably a Brahmin, unless you're in high school, in which case you'll probably be one in four years.
2. Dalits, who define status by "power, wealth, and sexual success" for men and "attractiveness and popularity" for women.
3. Helots, imported peasants who define status by "hard work, money and power" for men and "attractiveness, motherhood, and association with successful men" for women. (This caste is also not all that relevant for electoral analysis; Helots generally can't vote, and at least according to Moldbug, their children generally turn out Dalit.)
4. Optimates, the aristocratic caste, who pretty much aren't around anymore.
5. Vaisyas, who define status by social (especially church) participation and, for men, gainful employment. (Moldbug also lists family life, but I'm not so sure about that one; I know quite a few unmarried Vaisyas.)
To that list I'd add a sixth relevant caste: the Baniyas (or, perhaps, post-Optimates; the outline of that caste in the comments seems remarkably similar), who define status in terms of wealth and place/position of employment.
Now we can move on into the fun part: contemporary American partisan politics, or rather, the cultural conflict at the center of said politics. This is outlined here; again, I'll summarize. Broadly speaking, the Democrats are the party of Brahmins, Dalits, and Helots, and the Republicans are the party of Optimates and Vaisyas. This division is referred to as the BDH-OV conflict. (Baniyas, who aren't in Moldbug's analysis, are generally associated with the Republicans, but I suspect they'd have much less of a problem with changing their alliances than the other castes would. On the other hand, they tend historically to be more welcome on the OV side.) However, on top of that is the race divide: blacks and most Hispanics (specifically, those without ties to Cuba or Venezuela) generally vote Democratic, regardless of their caste.
We now have most of the picture assembled; we only need two more. The first is that humans are fundamentally social -- more specifically, tribal. We see things in terms of ingroups and outgroups. We like our guys to have as much power as possible, and we don't like Those Bastards, Over There getting their guys anywhere near the reins. (If you need any examples of this, consider the average high school. The nonconformist is a much rarer beast than we like to imagine, and when we do come across one, we send him to a couch and get a man with a long list of letters after his name to throw Latin-sounding terms and expensive pills at him. Alternatively, take an introductory class in social psychology.) The second is that these castes generally do not share cultures: not all Brahmins in America are members of the Doctor Who fandom, but you'd need to fling the decimal point pretty far to the left to get the percentage of members of the Doctor Who fandom who are not Brahmins. There are better markers of caste allegiance than that, of course. In fact, there are two that are very reliable markers of Brahmindom: abortion and gay marriage. Most Brahmins support them, and most non-Brahmins do not. (I think this even cuts across racial lines, although I've met very few black or Hispanic Brahmins, so I can't say with much certainty, although stories abound of black Democrats whipping the white ones into all sorts of righteous fury by supposedly leading to the defeat of gay marriage laws.)
And guess what Santorum is known for? Opposition to abortion and gay marriage.
Not only that, but he's in the right tribe. Romney's financial background, family history, and coastal/liberal background mark him off as a clear non-Vaisya; more specifically, a Baniya. (The label 'post-Optimate' for the latter caste may make that connection more obvious. His father, remember, was both a businessman and a governor.) Not only that, but he refuses to do what George W. Bush did: hide his background and run straight OV. Gingrich is an academic (Brahmin alert!) with a reputation for supporting large government projects (Brahmin alert!) and being a delusional wanker (Br--ah, damn, I suppose I don't really get to call that one.), and Paul is a raging ideologue. For Vaisyas who are feeling more tribal than pragmatic and were alive during the Gingrich years, Santorum is really the only option.
As for the man himself, his wrongness on any issue directly correlates with how much control the White House has over that issue: he's not a loot-and-pillage neoliberal*, but he's a Wilsonian, which puts him right out as far as I'm concerned.
* I'm not even sure if he's a liberal (in the sense of the word that's used everywhere outside American partisan politics) at all. What a refreshing change that would be from the parade of wankers we normally get in this country, trumpeting ideas so self-evidently wrong that one cannot avoid being reminded of Monty Python, if only he didn't want to conquer the earth in the name of said ideas.