Editor's note: William Howell is the Sydney Stein professor in American politics at the University of Chicago.
(CNN) -- With Rick Perry instantly assuming the top spot among candidates for the Republican presidential nomination, it is no surprise that nearly every other man and woman on stage with him at last night's debate took aim.
Citing his executive order mandating inoculations against a sexually transmitted disease that can lead to cervical cancer, Michele Bachmann expressed outrage that Perry, she said, had "forced" "government injections" of 12-year-old girls. Jon Huntsman archly claimed that Perry's skepticism about a border fence was nothing short of "treasonous." Ron Paul accused Perry, his fellow Texan, of raising his taxes, increasing the state's debt, and expanding the state government's workforce.
The most extended critique of Perry, though, came right out of the gate, from Mitt Romney. Romney accused Perry of trying to scare senior citizens and ruin the Republican Party by calling Social Security unconstitutional, a Ponzi scheme and a failure.
Of all the jabs at Perry last night, these mattered most. Social Security stands out as the most important topic generating substantive disagreement among the top Republican contenders. In a debate that vacillated between broad principles of national defense and the rule of law on the one hand, and small government initiatives conducted at the state and federal levels on the other, this particular exchange featured disagreements about one of the oldest, largest and most popular entitlement programs in the country.
Just as important, the exchange between Romney and Perry over Social Security underscored their competing views about the meaning of limited government. In their first sentences about entitlement reform, these candidates stand together for reducing waste, slashing fraud and trimming the federal government. It is in the second and third sentences, though, that the differences become clearer.
For Perry, power should devolve to the states, and the federal government should maintain operations over only those policy domains for which it has a clear constitutional responsibility. As he noted in his opening remarks last night, the very reason he wants to be president is to make Washington less consequential in the lives of Americans. When asked later about Social Security, Perry went out of his way to offer assurances that the benefits for current and near retirees would remain intact -- "slam dunk, guaranteed," as he put it. But he persisted in characterizing the issue of Social Security reform primarily as one of increased state control. "The issue is, are there ways to move the states into Social Security, for state employees or for retirees."
Romney, by contrast, has proposed a variety of measures that in his view will shore up Social Security without either increasing taxes or reducing the federal government's involvement. Though he didn't lay them out Monday night, the components of Romney's plan include the raising of the retirement age, the creation of personal retirement investment accounts for younger workers, and the indexing of benefits to prices rather than wages, as is currently done.
Between the two at last night's debate, Perry was the clear audience favorite. But should Perry win the nomination, we can expect President Obama to pick up right where Romney left off on Social Security. On the other hand, if Romney can survive a Republican primary season in which the median voter is vastly more conservative than in the general election, then the terms of next year's debate about Social Security will be vastly more conventional -- to the great relief of moderates and frustration of the tea party sponsors of last night's debate.
The opinions expressed in this commentary are solely those of William Howell.