Sorry, Newt. Sorry, Rick. Sorry, other Rick. And sorry, Ron, although you’re probably used to being ignored by now.

The only person who mattered at Monday night’s debate in Myrtle Beach, S.C.—the only candidate whose performance is even worth evaluating—was Mitt Romney.

The reason is simple: the Republican nominating contest has ceased to be a contest at all and has instead become a perfunctory Hell Week hazing ritual for a guy who’s already on a glide path to the nomination.

Sure, Gingrich got off a good line about unemployment insurance: that “99 weeks,” the current eligibility period, is enough time to get “an associate degree.” Sure, Santorum managed to lure Romney into an unflattering scuffle over felon voting rights. Sure, Paul won the “who will lower income taxes the most?” debate by saying he’d just lower them to zero.

But none of this will change the fact that Romney won Iowa; won New Hampshire; is poised (PDF) to win South Carolina and Florida; and is ahead in the national polls, which will largely determine the outcome on Super Tuesday, by as much as 25 percent. Unless the Mittster suddenly blurts out that he was brainwashed into opposing gay marriage or something, he’s going to be running against Barack Obama in the fall.

Which brings us back to Monday night’s so-called debate. As a skirmish in the broader war for the Republican nomination, it was inconsequential; the war is all but over. But it did serve a deeper purpose. It gave us our best glimpse yet of what kind of general-election candidate Romney will be.

His performance should worry conservatives. As usual, Romney was crisp and composed when answering the predictable questions about economic and foreign policy and reciting snippets of his stump speech. But he exposed a few real soft spots over the course of the 90-minute show.

The first was Bain Capital. For all Perry and Gingrich’s sound and fury—all their talk of “vulture capitalism” and what not—both of them shied away from making the most effective argument against the Bain model: that it tends to destroy jobs as often as it creates them. Their circumspection allowed Romney simply to equate what he did with “free enterprise” and parrot misleading claims about the “120,000 jobs” some old Bain companies have “ended up [with] today.”

As much as Perry and Gingrich may have wanted to expose Romney’s record—“We cannot fire our nominee in September,” Perry said. “We need to know now”—they failed to do so. Monday night was one of Romney’s first opportunities to make voters comfortable with his discomfiting Bain background, and he didn’t even have to try. Which is unfortunate for him, and for the GOP, because he needs all the practice he can get: attacks on Bain will be much more potent in the fall, and Obama will be a much more effective attacker.

Romney’s second vulnerability: his income-tax returns, which Perry and debate moderator Kelly Evans pressed him to release. If and when the documents are made public, they are likely to show that the former Massachusetts governor collects millions of dollars in investment income and employs sophisticated tax strategies to keep his rate around 14 percent, which probably won’t endear him to middle-class voters.

But Romney’s real problem, at least Monday night, wasn’t the content of his returns; it was the way he reacted to being asked to release them. At first, he seemed to cave. “I hadn’t planned on releasing tax records,” he said. “But you know, if—if that’s been the tradition, then I’m not opposed to doing that.” But then he hedged. “Time will tell,” he added. “But I anticipate that most likely I’m going to get asked to do that around the April time period, and I’ll keep that open.”

Romney continued to stammer and stumble for another minute or two—“I have nothing in, in them that, to suggest there’s any problem, and I’m happy to do so” was quickly followed by “I sort of feel like we’re, we’re showing a lot of exposure at this point”—but the damage was done. Given an opportunity to take a clear, bold stand, Romney instead reinforced his image as an unprincipled, untrustworthy politician—in real time—and reminded voters how uncomfortable he is with unscripted moments (of which there will be plenty between now and November).

Romney’s final weakness: his approach to Obama. As Paul Krugman has pointed out, Romney seems unusually keen (for a major-party presidential nominee) on spouting transparent, easily disprovable lies about Obama’s record, and Monday night was no exception. “We have a president in office three years,” Romney said, “and he does not have a jobs plan yet.” Except that he does: it’s called the American Jobs Act, and it was introduced last September.

Likewise, Romney later claimed that “this president has opened up no new markets for American goods around the world in his three years,” when last October Obama revived Bush-era free-trade pacts with South Korea, Panama, and Colombia—“the most significant expansion of trade relations in nearly two decades,” according to the Associated Press. Suffice it to say that while this “making stuff up” strategy works just fine on Fox News, where lies about Obama often go unchallenged, it risks eventually making Romney look even more slippery than he does right now. Swing voters are not so eagerly fooled.

So far, Romney has had a rather easy run of it; he’s likely to wrap up the Republican nomination by the end of the month. But the prize fight is looming, and it won’t be nearly as painless as his primary cakewalk. Given his performance in Myrtle Beach—and the general-election vulnerabilities it exposed—Romney should probably be wishing for a few more weeks of intraparty sparring before the real battle finally, inevitably, begins.