Movement conservatives probably haven't actually succeeded at guarding against that scenario. Based on the 2008 and 2012 cycles, they put a lot of emphasis on vice-presidential selection, though the VP has little ability to guide policy. They also demand red-meat rhetoric as if it's binding. They ought to worry more about are the tradeoffs Romney is likely to make if elected.
But they are too invested in his getting elected to pin him down.
Their difficult spot is illustrated nicely by the recent kerfuffle over Romney's tax and spending promises. Presidential candidates "lie about each other, they lie about themselves, they lie
about issues they know intimately, and they lie about issues they
barely understand," Jack Shafer recently wrote in his Reuters column. "If either presidential candidate met you, he'd tell you a lie within 15
seconds of shaking your hand, and if he knew he were going to meet your
mother, he'd invent a special set of lies for her. Politicians lie not
because they're wicked -- though some are -- but because they've learned
that political markets rarely reward honest campaigners."
That's the truth.
Were movement conservative opinion-makers intent on informing rather than coddling or manipulating the rank-and-file, they'd have no problem acknowledging that, like Obama, Romney tells campaign lies. He insists, for example, that he's going to balance the federal budget, cut the deficit, spend more on the military, repeal the estate and alternative minimum taxes, keep in place the home-mortgage-interest deduction, broaden the tax base, and refrain from raising taxes on the middle class. In fact, it's wildly implausible to think he can do all of those things*, but because the conservative movement won't acknowledge as much, it is unable to discuss or to exert pressure about which promises get kept.
In a way this is rational. Some folks at the Heritage Foundation, AEI, and National Review figure that even if Romney is lying, they'll prefer his leadership to whatever Obama would do, so getting him elected is much more important than leveling with the public about his fact-fudging. Some movement conservative pundits and think-tankers cover for his deceptions with outright hackery. More often, they write content that is technically accurate, but framed in a way that only makes sense if the intention is to obscure the fact that Romney won't be able to do everything that he is promising (and no one knows exactly which promises he'll end up breaking).
The conservative movement's election-driven, rather than truth-driven, approach may well help Republicans get elected sometimes. But it also explains how, once in office, things turn out as they did during the Bush Administration. Of course Bush responded to electoral incentives to push something like Medicare Part D, rather than staying true to fiscal conservatism -- the conservative movement was so invested in his political success, and so uninterested in fiscally prudent governance, that everyone from Paul Ryan to hackish think-tank interns were ready to give cover for any budget-busting idea Karl Rove deemed a good idea. In the long run, the dearth of truth-driven conservative opinion-making during the Bush Administration did great harm to the movement.