Exactly four weeks from today, Iowa Republicans will attend their party caucuses. A new Washington Post/ABC News Poll confirms what two polls over the weekend said — that Newt Gingrich has built a substantial lead, with Mitt Romney losing ground and basically tied with Ron Paul. So what is Romney doing in Iowa? Why doesn’t he, as Reid Wilson suggests today, just bow out of Iowa and wait for New Hampshire, a state that is far better suited to Romney’s charms?

He can’t. 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.

And that’s why Romney is stuck in Iowa. If the week between Iowa and New Hampshire is full of talk of , say, a Perry surge, then the game changes in a way that’s very dangerous for the Mittster. An intense blast of positive press following a surprise third-place finish in Iowa could instantly revive Perry’s candidacy. So Romney will compete hard over the next four weeks. If he winds up barely scraping into third place, everyone is going to call it a huge loss for him — but I suspect that Team Romney will be inwardly just fine with that result. But anything worse risks a real disaster — and that’s why Iowa remains absolutely critical to Romney’s hopes.