There clearly has been some movement in the GOP presidential primary. Using RealClearPolitics national polling averages, Mitt Romney is now narrowly in first place, with Herman Cain slipping to second and Newt Gingrich moving up strongly to third. Texas Gov. Rick Perry is now a distant fourth, followed by Rep. Ron Paul (Tex.).

Gingrich seems to be making the same strong ascent in the polls that Rep. Michele Bachmann (Minn.) and then Perry and then Cain have each made at some point in the race. It remains to be seen whether Gingrich can continue to accumulate voters from Cain and Perry, or whether these three will divide up the lion share of the not-Romney vote.

There are several takeaways from this shift. First, the progress in the polls generally matches the debate performances of the not-Romney contenders. Less-than-impressive showings by Perry and then Cain seemed to trigger their downward slide in the polls.

Second, as the poll numbers for each of these not-Romney candidates increased, so did the scrutiny, and eventually they faltered for want of gravitas.

Third, we don’t know the exact reason for Cain’s decline. Was it that the harassment allegations finally sunk in? Or was it the lousy debate performances and his lack of knowledge on the issues? Probably some of all of those, it is logical to assume. As Cain’s luster as a conservative standard-bearer faded, voters felt less need to “defend” him from the harassment allegations.

And finally, Romney remains exceptionally steady, neither losing ground nor adding to his base of support. Hi supporters aren’t going anywhere else, but the not-Romney voters haven’t yet thrown in the towel in their quest for an alternative to him.

The next question in the race will be whether Gingrich can do any better than prior challengers in defending against the onslaught that is sure to come. Other contenders will now be targeting him in the debates. In some regards, the substantive arguments against him are the same as those against Romney and Perry. Both Gingrich and Romney, for example, favored the individual mandate. Both Gingrich and Perry have taken moderate stances on immigration. But Gingrich’s real weaknesses, as I’ve noted before, are his track record as a leader and his ethical failings. Will his opponents have the nerve to go after those?

Another factor is worth noting but has been largely ignored by the punditocracy: For a candidate who fancies himself as a policy wonk, Gingrich is remarkably good a critiquing others and poor at coming up with his own policy agenda. He’s put forth no tax plan, no spending plan and no jobs plan. He bludgeoned Rep. Paul Ryan (R-Wis.) on Medicare reform but hasn’t shown us his own comprehensive entitlement solution. It will be interesting to see if, when challenged, Gingrich replies with characteristic testiness or begins to formulate plans that look like smart conservative reforms.

Finally, it’s important to look at the early states to see where Gingrich could win. He’s made impressive speeches and spent time in Iowa (pandering along the way on ethanol), but his organization there is weak. Can he turn out his voters on caucus night? Moreover, it’s not clear that evangelical Christians, a critical voting bloc for the not-Romney contenders, will feel comfortable publicly embracing the thrice-married, ethics-challenged former speaker of the House.

In sum, Gingrich has many strengths the other not-Romney candidates lacked but also a bushelful of weaknesses. It should be an interesting couple of months.