Who says so?
Knowledgeable presidential politicians such as John McCain and other Republican grandees. That's who. They almost all say that. Why they say that I don't know.
Romeny has not been able to get past the 25% support mark in this campaign. He was so disliked as governor of Massachusetts that he decided not to run for re-election as he knew he'd lose. I read recently that Romney has run in 22 different elections. He has lost 17. Romney is batting 230 in the election game That's fact, not prediction.
So, why does the Republican Establishment declare Romney to be electable? It can't be because they hate Gingrich; they held this position from the beginning, before Gingrich's rise. They're not stupid; it's not a deathwish. It makes no sense at all.
Based on history, Romney is UNelectable; he's simply not liked by voters; he lacks the requisite charisma. He ain't got it. But, just like the statists they are, the Republican grandees know what's best for its voters. They forced Bob Dole and John McCain on the party faithful and are trying again with Romney. I don't think it will work this time around either.