COLUMBIA, S.C. — Newt Gingrich turned his fire on Representative Ron Paul of Texas on Friday, saying that his Republican opponent had to answer for political and investment newsletters that included racist, anti-gay and anti-Israel passages that Mr. Paul has disavowed.

Mr. Gingrich also sharply criticized Mr. Paul for what he said were his isolationist views on foreign policy. The pointed comments suggested a new dynamic in the presidential primary race, with Mr. Paul as a new and enticing target. His fortunes have risen in Iowa, scrambling the field as some polls suggest that Mr. Paul could pull off a victory in the caucuses on Jan. 3. But in recent days, he has come under increasing scrutiny for offensive passages in newsletters that bore his name, although he has denied writing or approving them.

“These things are really nasty, and he didn’t know about it?” Mr. Gingrich said to reporters after a town-hall-style meeting here.

At the same time, Mr. Gingrich refrained from criticizing Mitt Romney, with whom he has frequently sparred, calling him, at worst, “a Massachusetts moderate.”