A new ad on behalf of Mitt Romney pokes fun at Newt Gingrich, and says he is exaggerating his relationship with Ronald Reagan.

Mr. Gingrich frequently links himself to the former president, who is revered by many as the embodiment of modern conservatism — even as some Republicans question Mr. Gingrich’s conservative credentials.

The new ad asserts that Mr. Reagan, who died in 2004, did not return the love....