On the surface, the new (and final) Des Moines Register poll of likely Iowa caucus-goers basically corroborates polling from Rasmussen and NBC/Marist earlier in the week, which showed Mitt Romney slightly edging Ron Paul, while Rick Santorum gained enough ground to move into third place.  The numbers are almost identical, at 24/22/15 — but it’s not the topline that is the real news.  The four-day poll of 602 likely caucus-goers, which is a larger sample than NBC/Marist but smaller than Rasmussen’s single-day survey, had significant change in the final two days of polling on Thursday and Friday:

But the four-day results don’t reflect just how quickly momentum is shifting in a race that has remained highly fluid for months. If the final two days of polling are considered separately, Santorum rises to second place, with 21 percent, pushing Paul to third, at 18 percent. Romney remains the same, at 24 percent.

“Momentum’s name is Rick Santorum,” said the Register’s pollster, J. Ann Selzer.

Another sign of the race’s volatility: 41 percent of likely caucusgoers say they could still be persuaded to change their minds.

Newt Gingrich fell further off the pace to 12%.  Rick Perry, who had improved in the earlier two polls, scored a little lower in the DMR survey at 11%, and Michele Bachmann dropped to 7%.  Assuming that this is how the final caucus numbers turn out, it’s likely to be Bachmann’s last stand.  Perry and Gingrich will almost certainly have enough resources and energy to continue to South Carolina; Perry’s team announced that it would skip New Hampshire altogether, not exactly surprising as Perry hasn’t had much traction in the Granite State at all during this cycle, not even when Perry rode high in national polling.  Gingrich has polled better and might not be able to pass up a chance to try to dent Romney’s momentum coming out of a potential Iowa win.

Can Santorum win Iowa?  Rep. Steve King was skeptical of the idea, but Santorum’s proximity makes it at least a reasonable possibility.  What then?  Santorum won’t get much support at all in New Hampshire, so he’d likely take a page from Perry’s book and spend the week in South Carolina, hoping to build momentum with evangelicals and conservatives.  The last polls from South Carolina showed Gingrich firmly in command, but they were from two weeks ago, too, before Gingrich’s slide began in earnest.   Santorum’s average in the state from August to mid-December on Real Clear Politics put him dead last, even below Jon Huntsman.  Winning South Carolina would be a tall order for Santorum, and yet a failure to do so leaves Santorum with few paths to even being competitive through Super Tuesday in March, with states like Nevada, Michigan, and Florida on the schedule in between.  But if Santorum could pull off a miracle win in South Carolina, then conservative donors might flock to his banner as the sole remaining Not Romney with a viable shot at beating Mitt, even if still a long shot.

For Romney, though, the rise of Santorum and the Paul Stall is good news.  He’d be more threatened by Gingrich or Perry, and his steadiness in Iowa shows that he has staying power.  That plus his campaign resources should allow him to easily outlast Santorum, unless a Santorum surge forces everyone else out of the race early.  However, if Romney wins Iowa and then New Hampshire, that might be enough even for South Carolinians to consider the virtues of wrapping up the nomination fight early, regardless of how well Santorum does.

Update: A couple of readers wonder why I didn’t mention that Perry has gained five points since the last DMR poll as a sign of an imminent Perry surge.  Here’s why: Perry’s still only at 11%, there are four candidates polling ahead of him, Santorum has gained nine points since the last DMR poll and fifteen if you just look at the last two days of the survey, and there are only two days left.  I think that just about covers the Perry surge.  I’m not saying he might not stage a surprise on Tuesday, but clearly Perry isn’t the big news in this poll.