Why should Mitt Romney be worried about the Newt Gingrich surge in Iowa? Because it could threaten his dominance in New Hampshire.

Greg Sargent writes at the Plum Line blog that Iowa remains "absolutely critical" to Romney's hopes, and that's why he's going all-in in the Hawkeye State.

What Romney’s campaign seems to understand, and why he has fully committed to Iowa, is that the important thing isn’t winning Iowa. It’s winning the week after Iowa — the week between the Iowa caucuses and the New Hampshire primary — during which the media will decide what the Iowa results meant, with far reaching implications for New Hampshire, which is key to Romney’s strategy.

If the polls hold, the spin coming out of Iowa surely will be that Gingrich leads a two-person race. The press will continue to treat Ron Paul as a sideshow and ignore him, and the rest of the field will be starved for oxygen and rapidly drop out. That’s not a bad result for Romney going forward; while Gingrich does have some strengths, Romney will have solid advantages in money, in support from Republican opinion leaders, and in organization, and has a nice, fat, opposition research file on Gingrich with plenty of time to use it.

But while Romney could easily come from behind and close the gap — Nate Silver reminds us that significant late shifts in Iowa are very possible — it’s also not hard to believe that Romney could drop out of the top three entirely. The next group of candidates includes Rick Perry, Michele Bachmann, and Rick Santorum, and they’re tightly bunched about ten points behind Romney and Paul. If Romney does fall behind one of those others, that really does change things. For Romney it would be seen as a major defeat. What’s more, the press, always on the lookout for a new story, would probably devote quite a bit of attention to a Perry or Bachmann comeback or a Santorum surprise.