If he's the nominee he will beat obama. The only Pub running for the nomination who can't beat obama is Romney.
The biggest risk about Newt in the general is that he regularly manages to tick off (and demoralize) conservatives (although, so far, many seem willing to give him a pass on anything that occurs) and at the same time he’s deeply hated by the many rank-and-file Dems.
Did you hear Newt’s buddy Al Sharpton today? Gingrich actually went on an “Eductation Tour” with Al Sharpton in 2009 to promote Obama’s policies. Today Sharpton was blasting Gingrich as “heartless” and “always blaming the poor kids.” So, Newt ticked off conservatives with that stunt and what did he even get for it? As predicted, nada.
Anyone who was politically sentient during the ‘90’s has a pretty well-formed opinion on Newt. For many Dems, the effect on them of having Newt nominated for President would be like the effect it would have on the GOP base is Nancy Pelosi were nominated for President. There would be crawling over broken glass en masse to vote against him/her.
It’s a voter intensity thing and it’s hard to say how it would play out.