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.

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.