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Charles Dharapak, Associated Press
Republican presidential candidate, former Massachusetts Gov. Mitt Romney campaigns at Andrews Field House at Wofford College in Spartanburg, S.C., Wednesday, Jan. 18, 2012.

COLUMBIA, S.C. — Mitt Romney's Republican rivals are intensifying their efforts to erode if not eliminate his standing as the man most likely to defeat President Barack Obama this fall.

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They're often stressing their own prospects over his in the final few days before South Carolina's potentially decisive weekend primary.

The stepped-up challenge to Romney's electability comes in paid television advertising, campaign appearances and the first of two pre-primary debates. It appears aimed at one of Romney's principal strengths in the Iowa caucuses and the New Hampshire primary.

Voters in both contests said they prized a candidate's ability to beat Obama over a candidate's conservative credentials, a preference that helped the former Massachusetts governor to his pair of victories.