Former Massachusetts governor Mitt Romney’s back-to-back wins in Iowa and New Hampshire over the past 16 days have established him as the heavy favorite for the Republican presidential nomination in 2012.

But, as the campaign shifts to South Carolina, Romney faces by far his toughest electoral test. That much has become cemented — or maybe congealed — as conventional wisdom.

Republican presidential candidate, former Massachusetts Gov. Mitt Romney, and South Carolina Gov. Nikki Haley campaign at The Hall at Senate’s End, in Columbia, S.C., Wednesday, Jan. 11, 2012. (AP Photo/Charles Dharapak)

First, South Carolina’s electorate is simply less friendly for Romney than either Iowa or New Hampshire.

Exit polling from the votes earlier this year showed that 57 percent of Iowa Republican caucus-goers considered themselves “born again/evangelicals”; just 22 percent of New Hampshire GOP primary participants said they were “born again/evangelical”.

In exit polling from the 2008 South Carolina primary, 60 percent of primary voters said they were evangelicals and Romney won just 11 percent of their votes — below the 15 percent he got statewide. (Among non-born agains, Romney took 20 percent — overperforming his statewide number.)

Evangelical voters have long been Romney’s most difficult voting bloc within the Republican electorate due, almost certainly, to born agains skeptical (at best) view of Mormonism.

The electorate in South Carolina is not only more stocked with evangelical voters than Iowa or New Hampshire but it’s also at least as conservative.

In 2008, 69 percent of Republican primary voters said they were either very or somewhat conservative. In New Hampshire in 2012, just 53 percent of primary voters described themselves that way; in Iowa the number was 83 percent but remember that the definition of conservative in Iowa is likely different (and less conservative) than it is in South Carolina.

The second — and perhaps more challenging problem — is the amount he will likely need to grow his vote by in order to win South Carolina.

In 2008, Romney got 67,970 votes (15 percent) in the Palmetto State primary. Arizona Sen. John McCain won that race with 143,224 votes (33 percent).

Assuming roughly similar turnout — 431,000 — to 2008, that means Romney would need to more than double his vote share from four years ago in order to win. (The lowest recent vote total for a South Carolina primary winner was 124,904, which was what then Kansas Sen. Bob Dole took in victory in 1996.)

Possible? Of course. But a look back at Romney’s ability to grow his 2008 vote to date in the nomination fights suggests it might be tough to increase his vote total by more than 100 percent.

In Iowa, Romney got 66 more votes in 2012 than he did in 2008, an increase of .2 percent. In New Hampshire, he got 22,189 more votes in 2012 than 2008 — a 29.5 percent increase. Increasing vote by 100 percent (or more) is a far more difficult proposition.

Of course, Romney may not need to win South Carolina. The Florida primary is just 10 days after South Carolina and polling suggests Romney is a prohibitive favorite in the Sunshine State. And, even if Florida is somewhat rocky for Romney, he is a clear favorite in Arizona (large Mormon population) and Michigan (his home state). Both states vote on Feb. 28.

Given the hurdles he faces in South Carolina, if Romney is able to win there in nine days time he could — and should — seal the nomination.