A.B. Stoddard believes Santorum blew his opportunity to win the nomination:

Despite Romney’s overpowering resources and organization, Santorum’s potent argument — that the party could not throw the issue of healthcare away by nominating someone who had supported mandates — was his key to victory, but he threw it away.

There are many things wrong with this. Stoddard assumes that Romney’s voters, who tend to be less conservative and less ideological, would have responded favorably to the more ideological candidate if he had harped on a detail of Massachusetts health care legislation even more often than he did. This misunderstands how most of these voters decide on their candidates, and it overrates the importance of policy differences in shaping electoral outcomes. Another mistake is believing that attacking the individual mandate was Santorum’s strongest argument against Romney, which seems hard to believe. To cite just one example, Romney has always been more vulnerable on his support for the financial bailout, which is something that provokes strong and generally negative feelings among most Republican voters regardless of ideological leanings. Another flawed assumption is that Santorum “threw away” this health care argument because he became distracted by the various controversies over contraception and religion. Santorum continued to use his health care criticism of Romney all along, and Santorum made this argument to convey his broader message that Romney was too compromised to be the nominee. One problem for Santorum is that there have been enough Republican voters in the largest primaries that don’t find this broader message compelling.

Santorum consistently wins “very conservative” voters with his message, but he always struggles with the rest of the Republican electorate. Ideological and “very conservative” voters regard Romney’s support for an individual mandate at the state level as an unacceptable compromise, but this is not a view shared by a large part of the Republican electorate. To believe that hammering Romney on the individual mandate was Santorum’s “key to victory,” one has to imagine that Santorum could have won Michigan or Ohio if he just spent more time talking about this issue, and that doesn’t make sense. Criticizing Romney on the individual mandate is the sort of thing that some policy wonks and people in Washington think is a powerful political attack. The voters who care about this weren’t voting for Romney anyway, and for the most part Romney’s natural constituencies don’t care about it that much.

Stoddard continues:

Combined with his push for fewer business regulations and less taxation, Santorum’s message on ObamaCare was all he needed to win over enough Romney voters to close the gap and beat him. It was the key to his appeal to independent voters, and to those voters focused more on the economy than the social issues Santorum has built a reputation championing.

It doesn’t help Stoddard’s case that Santorum won independents in Ohio and virtually tied Romney with independents in Michigan. Santorum struggled more with registered Republicans in both states: he lost them by eleven points in Michigan and four points in Ohio. The gap he needed to close was with members of his own party.