So much for the 25% cap … or can we rely on this poll to make that determination?  Fox News takes a national poll and finds that Mitt Romney has jumped 17 points in five weeks among likely GOP primary voters nationwide, with Rick Santorum edging into a distant second over Newt Gingrich and Ron Paul.  However, the sample is questionable for the primary question, if solid on general-election and political issues.

Fox takes a sample of 906 registered voters (landline and cell), which breaks out into a D/R/I of 41/39/20, which gets a good balance between Democrats and Republicans but oversamples both at the expense of independents.  From this, the pollster gets 365 primary voters, which it claims as “likely voters,” even though this is the same subset of Republicans in the sample marked “registered voters.”  However, this is the same methodology used in the Fox poll series for the last several months, and about the average for sample sizes on the primary question, so the series can at least be expected to reflect trending.

In the beginning of December, Romney got 23% for second place to Newt Gingrich’s 36%.  Prior to that, Romney scored 22% in November, 20% in October, and so on; his highest level of support came in July and August at 26% in both months.  Now, however, Romney has suddenly spiked upward to 40%.  On the question of managing the economy — the issue that used to be the central battleground in the 2012 election — Romney scores 44% over Gingrich’s 17%.  He also beats Gingrich in the potential to work with Congress, 35/28, an issue  that Gingrich raises repeatedly.  Rick Santorum, however, wins recognition as the “true conservative” with 29% — but Romney comes in second at 15%, just ahead of Gingrich at 14% and Ron Paul at 13%.

Gingrich does win one title in this poll, however … Miss Non-Congeniality:

GOP voters perceive Newt Gingrich as running the most negative campaign in this year’s Republican presidential contest, according to a new poll on Monday.

Twenty-nine percent of registered, Republican voters said Gingrich’s campaign has been the “nastiest” this cycle, according to the national survey released by Fox News.

Mitt Romney leads the field for positive campaign perception at 27 percent.

Don’t forget that just four weeks ago, Gingrich was winning plaudits for his insistence on running a positive campaign.  That’s quite a turnaround, and an indication of just how much backfire Gingrich has taken with the Bain attacks.

While we consider that, don’t miss Barack Obama’s numbers in this poll, either.  After going into majority disapproval in December with a 44/51 approval rating, the President has come back a bit to 45/47.  His favorability remains surprisingly weak at 51/46, though, so he doesn’t have much more upside to explore.  There may be something to the argument that Republicans would be better served to wrap up the internecine fight quickly, but given how many months there are to go to the general election, it doesn’t appear to be a big worry at the moment.