Mitt Romney used a line at the last debate to minimize Newt’s claim to have worked with Ronald Reagan (transcipt, emphasis mine):

MR. ROMNEY: … I mean, Mr. Speaker, it was — it was — you talk about all the things you did with Ronald Reagan and — and — and the Reagan revolution and the jobs created during the Reagan years and so forth. I mean, I looked at the Reagan diary. You’re mentioned once in Ronald Reagan’s diary. And it’s — and in the diary, he says you had an idea in a meeting of — of young congressmen, and it wasn’t a very good idea, and he dismissed it. That — that’s the entire mention. And — I mean, he mentions George Bush a hundred times. He even mentions my dad once.

Romney stole the line of attack from Ann Coulter’s column, Newt helped formulate Christmas (emphasis mine):

In Ronald Reagan’s autobiography, “An American Life,” he writes extensively about supply-side economics. He cites Jack Kemp several times. He never mentions Newt Gingrich.

(However, in Reagan’s massive 784-page diary, Newt’s name does come up — once. On Jan. 3, 1983, Reagan wrote that he met with “a group of young Repub Congressmen,” and says that one of them, “Newt Gingrich,” proposed freezing federal spending at 1983 levels, which Reagan rejected out of hand because it would “cripple our defense program.”)