As Republicans leave Iowa, what happens now?

The ideal scenario for Mitt Romney in Iowa wasn’t just a victory — he also wanted the other candidates to do just well enough to stick around for a long while. Because the former governor’s support in his party is so limited, Romney benefits greatly from as large a field as possible — the more the anti-Romney vote is divided among many candidates, the easier it is for him.

As it turns out, this isn’t going Romney’s way, either.

Rick Perry fared poorly in Iowa — despite having spent more than $6 million in the state — garnering just 10.3% of the vote in a fifth-place finish. Going into Tuesday, the Texan and his team said they were headed for South Carolina no matter what. Last night, the forgetful governor announced, “With the voters’ decision tonight, I’ve decided to return to Texas, assess the results of tonight’s caucus, determine whether there is a path forward for myself in this race.”

That’s obviously not a formal withdrawal, but once a candidate talks about quitting, scraps his schedule, and heads home, it’s a safe bet he’s done.

And then there’s Michele Bachmann, who looked like a leading contender in Iowa as recently as August, but who finished a humiliating sixth last night, with just 5% of the vote. Last night, the right-wing congresswoman told supporters she “can and will defeat” President Obama, but when the AP asked Bachmann’s campaign manager whether the campaign would continue, he said, “I don’t know yet.”

NBC News’ James Novogrod reported this morning, meanwhile, that Bachmann “has apparently cancelled her South Carolina trip,” and will host a press conference at 11 a.m. eastern.

Also keep in mind, Jon Huntsman is betting his candidacy on New Hampshire. If he comes up short, as seems likely, the former Utah governor will probably step aside, too.

In other words, over the next week or so, the Republican presidential field may quickly go from seven candidates to four. That’s the exact opposite of what Romney was hoping for.

What’s more, not only are Perry and Bachmann supporters unlikely to shift to Romney — Rick Santorum is the likely beneficiary — but Newt Gingrich now appears desperate to destroy Romney in the coming days and weeks, subjecting the former governor to the kind of pressure he’s avoided up until now.

To be sure, Romney is still the frontrunner and the likely nominee. But a smaller field, more aggressive rivals, and shifting momentum may well make the process more difficult than he’d like.