There will be anti-Romney. Perry still best positioned.I don't think that's quite right, or at least I don't think it gets at how things play (of course, to be fair it's a tweet, so of course he didn't lay out the full case for what will happy; I'm just using it as an excuse).
Anyway: I think there's a very live possibility that there will be effectively be no anti-Romney -- that Romney will wrap things up in the next few weeks, and the primaries and caucuses will just be about making it official.
That's what appears to have happened on both sides in 2000. Wait, you're asking -- both sides? Yup. As I read it, the late McCain challenge to George W. Bush was really a sideshow, with Bush having already wrapped up the nomination by securing the backing of the overwhelming bulk of party actors (and facing attempted vetoes from none of them). Even when the nomination is settled, a rogue candidate can fight on, and might even win a few states (and on the Democratic side, can win some delegates), but it doesn't really have anything to do with actually nominating a candidate. That's easy to see when it happens after the winner has already accumulated enough delegates to win, but it can be just as true, I think, even in Iowa and New Hampshire.
So one possibility is that Romney wins early, but even if that's the case someone will at least finish second in Iowa and New Hampshire, and it's even possible that someone could win in one -- maybe even both -- of those states while still not having any plausible chance of being nominated. If Cain or Bachmann or Gingrich or Santorum or Paul or Roy Moore wins early but doesn't win any significant endorsements or support from party actors, odds are he or she will fade out rapidly, and Romney will be nominated easily.
So I'm not sure that Rick Perry at this point has the best chance of winding up challenging Romney in the early states. What I do think is that if Perry gets his campaign going enough to do that, then he'll have a solid shot of actually winding up the nominee. That's why I agree with Seth Masket that Perry is still viable. But if Perry succeeds in making himself unacceptable to Republicans, then it doesn't really matter who serves as the early-state anti-Romney, because whoever it is isn't likely to come anywhere close to being nominated.