Tomorrow’s debate in Mesa, Ariz., is a critical opportunity for Rick Santorum to demonstrate he’s not going to launch a futile quest to repeal the last 50 years of social history, and for Mitt Romney to demonstrate he is the most electable conservative in the race.

Santorum will no doubt be grilled on the batch of statements (old and new) that have come to light in the last couple of weeks. He must answer without rancor or defensiveness, and give the voters a preview for how he would defend himself in a general election. Screeching at the liberal media or claiming it’s all a big misunderstanding isn’t going to cut it. Without suggesting he’s not sincere in his beliefs, he needs to reassure voters that he understands the difference between his private views and his public agenda. (That will be a little tougher for Santorum than the average pol since he’s long advocated that values are essential in evaluating a candidate.) It is critical that he not snap or whine, but patiently explain himself and offer reassurance to those who consider him a social conservative stalwart but an indifferent free-marketeer.

In particular he’d be wise to explain that he understands women have served admirably and successfully in combat situations (and that he won’t go about reversing years of military policy expanding their roles) and work outside the home to improve their communities and countries, and yes, to seek personal affirmation just like men. His book has passages that suggest otherwise; he’d be wise to show he’s continued to evolve in his thinking and is not a time-warp-trapped Neanderthal. If he continues to misrepresent what he has written, he will pay a price with voters.

He’s also going to have to deal with the fiscal conservatives’ upset with his special manufacturing tax rate (zero). It’s not enough to say he’s delighted to take on the Wall Street Journal; he’s got to respond to the legitimate concerns of Tea Partyers, business people and limited-government advocates. Frankly, he’d be smart to dump the whole thing and talk about the rest of his economic agenda.

And finally, his record on Big Labor from the conservative perspective is atrocious. He should explain why he voted as he did, what he’s learned and how he would treat issues such as Davis-Bacon, free trade agreements, as well as right-to-work and striker replacement legislation.

As for Romney, now is the time to dispel the criticism that he’s all negative. His ads and campaign have been going hammer and tong against Santorum (which he should defend as evidence he’ll be no cream puff when it come to battling President Obama). But he has begun (at CPAC and on the stump) to remind voters of his own agenda. That agenda, he will need to argue to skeptics, is extraordinarily conservative (cut discretionary spending, reduce the federal workforce, develop domestic energy supplies, etc.) and far more bold than anything Obama has come up with (e.g., send Medicaid to the states, reform Medicare using a premium support model, raise the retirement age and index Social Security benefits).

He shouldn’t strive to thrill the base with conservative rhetoric. Many of them have vowed to be unimpressed no matter what he says. Rather, he’d be wise to explain he’s not a phony rhetorician. He’s not going to sweet talk them or pretend he’s been studying Burke from infancy. Instead, as he did at CPAC, he’d do better to explain how his family, business experience and service as a chief executive (of a state and the Olympics) have made him a conservative by experience and a competent conservative at that.

His “Washington insider” meme is both trite and unimpressive. His opponents’ lifetime in D.C. is not the problem per se. It is that they have missed critical experiences (running a business, overseeing a charitable foundation, making financial decisions, hiring and firing staff) that are in high demand now.

Moreover, he needs to be more specific in explaining why his executive leadership skills put him in a unique position to confront Obama (who, for example, has never presented an entitlement reform plan to Congress) and then to govern. Rather than dismiss lawmakers as people who only have to worry about getting a bill out of committee, he should be blunt and detailed: His opponents have never hired hundreds (if not thousands) of people, prioritized an agenda and gotten it through a hostile legislature and made pressure-packed decisions that a president or other executive must continually make.

This is the last debate before the Arizona and Michigan primaries on Feb. 28 and the slew of contests on Super Tuesday. The impression the candidates leave, and the concerns they raise or put to bed, will be critical in deciding the trajectory of the race.