Last night’s debate sketched out the ground that fight for the nomination will be fought on. Unless something unexpected shakes the race up (the entry of another candidate is the most obvious possibility), this is a race between Rick Perry and Mitt Romney. (Michele Bachmann has sunk in the polls and shed top staffers since Perry entered the race and, as she faded into the background last night, looked unlikely to recover.)

Perry’s case involves running to the right and — with the lower-tier candidates often backing him up — attacking Romney. Perry went after Romney with gusto, calling the Massachusetts health care plan “a great opportunity for us as a people to see what will not work” and telling Romney that “Michael Dukakis created jobs three times faster than you did, Mitt.” (Perry is clearly an enthusiastic brawler; when Ron Paul took a shot at him, he ignored the conventional “don’t punch down” wisdom and went ahead and took a shot back.)

Romney’s case is an electability case: He argues that Perry will turn off general election voters. In this debate it was about Social Security: “Our nominee has to be someone who isn’t committed to abolishing Social Security, but who is committed to saving Social Security,” said Romney. Romney was misstating Perry’s position, but Perry didn’t do a great job of correcting him, missing the opportunity to emphasize that he opposes changes to the system that will affect current beneficiaries. The Romney campaign sent out its spin that this means Perry can’t win the presidency.

Look for this pattern to be repeated on other issues. There are plenty of areas where Perry can attack Romney from the right (particularly the numerous issues where Romney has changed his position over the years), and plenty of areas where Romney can argue that Perry will have general election liabilities.

Interestingly, Jon Huntsman sent out a release today attacking Romney from the left, arguing that he and Perry are “two peas in a pod” on Social Security. I’m sure this will please Huntsman’s base, which consists of the reporters on his campaign bus, but it’s a gift to Romney. The difficulty of arguing a rival is too far right to be electable is that, done wrong, it can make you seem too far left to be acceptable to Republicans. Huntsman’s longshot strategy of targeting New Hampshire independents establishes an ideological guardrail that helps protect Romney from that trap. To a certain extent, this holds on the right, too: If Perry is such an extremist, a voter might wonder, why are Michele Bachmann and Rick Santorum attacking him for being a squish? That only works to Perry’s advantage if the attacks from the right don’t resonate too deeply, of course. But for now, the dynamics of the broader field seem to reinforce rather than challenge the salience of the Perry vs. Romney fight.