A next-day analysis of the Republican presidential primaries in Michigan and Arizona won by Mitt Romney underscores one of his weaknesses with his party's base, especially with the ascent of his now-chief rival Rick Santorum: he fares more poorly with born-again and evangelical voters than with non-evangelicals.

It was particularly apparent in Michigan, according to exit poll results analyzed by the Pew Research Center's Forum on Religion & Public Life. Voters who were polled could describe themselves as born-again or evangelical, regardless of their religious affiliation.

Fifty-one percent of self-described evangelicals in Michigan's primary voted for Santorum, Romney's top rival for the GOP nomination.

Romney, who is Mormon, got 35 percent of that part of the evangelical vote. Thirty-nine percent of Republicans who marked ballots in Michigan Tuesday were white, evangelical and Protestant, Pew reported.