"It would be a disaster"

This seems to best summarize the GOP's horror at the thought of Gingrich as the nominee:

It could happen, and it would be a disaster. All of us who were around and saw how he operated as speaker — there’s no one who’s not appalled by the prospect of what could happen.... He could win the presidency if there’s a way to win with 45 percent — a second recession or a third-party candidate. The immediate worry is him winning the nomination and losing the election, tanking candidates down-ballot.

PM Carpenter weighs in:

If Republicans nominate Romney, as is likely, the far right, subsequent to Romney's annihilation in the general election, will instantly begin agitating for a "true conservative" to run in 2016. And that means another batch of Cains, Bachmanns, Perrys, Palins, and so on. Their party will suffer through another dreadful four years of moving farther and farther right, while the electorate moves farther and farther away from what should be an authentically conservative party -- which a healthy, two-party American democracy requires.

If, however, the party nominates Gingrich, the right, post-2012, will be deprived of its now-incessant complaint that the party too often nominates a losing "moderate." It's true that Gingrich has flipped or wobbled on several issues of key interest to the far right, but no one among the pseudo- or ultra-conservative ranks will be able to convincingly deny that the party's 2012 candidate was himself, in the personage of Newt Gingrich, in so many primal ways, an ultraconservative. And with Gingrich's resounding defeat, that brand of ultraconservatism will have proven itself to be altogether electorally unviable.