Morning Edition from NPR

On the campaign trail, GOP presidential candidate Rick Santorum often discusses his opposition to abortion rights and gay marriage. That message served him well in Iowa with its large contingent of evangelical voters. Christian conservatives are also dominant in South Carolina, which votes Saturday. Santorum hopes to repeat his Iowa performance, but he's been struggling to keep pace in polls.

GOP presidential campaigns and superPACs have been spending millions of dollars on TV and radio advertising ahead of Saturday's South Carolina primary. While the negative superPAC ads air, the candidates are delivering a more positive message.

David Greene has business news.

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DAVID GREENE, HOST:

As Mara just mentioned, the debate last night opened with a question about a claim made by Gingrich's ex-wife. Well, Marianne Gingrich gave her exclusive interview to ABC's "Nightline," putting the candidate's personal life in the spotlight once again. NPR's Tamara Keith has more.

TAMARA KEITH, BYLINE: The interview was incredibly hyped, in part because Marianne Gingrich has been silent so far on her ex-husband's presidential candidacy. This was her first television interview since their 1999 divorce.

Saturday is South Carolina's Republican presidential primary. It's also the second anniversary of the Supreme Court's famous Citizens United decision.

That's the case that allows corporations to explicitly support or attack specific candidates. The day will be marked with attack ads — and protests.

The Republican presidential race has covered just three states so far. And superPACs linked to candidates Mitt Romney, Newt Gingrich, Rick Santorum and Ron Paul have spent a total of $20 million. They're feeding voters a heavy diet of negativity.