The promise of his triumphs in the Deep South last week was that they would give Santorum the true one-on-one race with Romney that he’s long sought, unifying and energizing conservatives and propelling him to an upset win in Illinois. Under this scenario, Illinois would then have kicked off a late primary season charge in which Santorum would have big states that were previously thought to be off-limits to him, causing Romney to melt down and allowing Santorum to claim an outright delegate majority by June.

But that’s not going to happen. Santorum’s 12-point loss in Illinois was defined by the same demographic realities that have shaped every Republican contest so far, with Romney cleaning up with higher-income suburbanites and non-evangelicals, and Santorum running well with white evangelicals and lower-income rural voters. Moreover, Santorum’s lack of organization allowed Romney to squeeze some extra delegates out of the state. We’ve seen this story before.

So the question now is how long Santorum can stay in this race. Clearly, he’d like to deny Romney a primary season delegate majority and spend the pre-convention summer months maneuvering to … do something. (Win the nomination himself? Force Romney into a ticket-sharing deal? Throw his delegates behind a different candidate?)

It’s very unlikely it will come to this; even if Romney doesn’t quite reach 1,144 during the primaries, he’s likely to finish very close to it – close enough that a handful of the GOP’s version of superdelegates would probably put him over the top without much trouble. More likely, Romney will break 1,144 sometime in June and that will be that.

But it could still be dicey, given that several states that aren’t demographically hospitable to Romney still remain. So Santorum probably has a good incentive to keep at it for a while, to see if the demographic patterns that have governed this race hold – and if they will be enough to prevent Romney from clearing the delegate threshold.

Again, it’s a long shot that Santorum could accomplish even this, but he won’t be forced to acknowledge it in the near future — unless there’s a decisive break in the demographic patterns, one that leads Santorum to fare poorly in a state he’s supposed to win. At that point, the psychology of the race would change dramatically, potentially compelling Santorum to raise the white flag.

Where could Santorum suffer such a loss? Here are the most likely possibilities:

1. This Saturday: Louisiana seems as tailor-made for Santorum as Illinois was for Romney – a more rural, lower-income Republican electorate with a giant chunk of white evangelicals (59 percent of the 2008 primary electorate). A poll released Tuesday has Santorum leading by double-digits, and in his two presidential campaigns Romney has never won in the South (except for Virginia, where he and Ron Paul were the only names on the ballot, and the New South state of Florida, where he was still crushed in the culturally southern Panhandle region). Anything short of a victory for Santorum on Saturday would represent a major psychological blow.

2. April 3: Santorum will lose the District of Columbia, where he failed to make the ballot, and probably Maryland too, a state that is demographically ill-suited to him. To avoid being swept, Santorum will need to prevail in the day’s other contest, in Wisconsin. On paper, he should be competitive – the state is similar to Ohio and Michigan, where Santorum nearly pulled off upsets. The media may hype this as a do-or-die contest for Santorum, and Romney figures to flood the state with resources in an effort to kill off his pesky rival once and for all. A decisive defeat here could theoretically force Santorum to reevaluate his candidacy. But if he loses by a tiny margin, it might not really signal anything, since the outcome should be within a few points either way, based on demographics. In other words, the media would probably play up a Wisconsin defeat as the final death knell for Santorum, but there’s a good chance he’ll ignore it and focus on …

3. April 24: Santorum will likely lose four states on this day – New York, Rhode Island, Connecticut and Delaware. But the other big prize is Pennsylvania, Santorum’s home state. So he’ll be under enormous pressure to win it. The last active White House candidate to lose his or her home state after winning elsewhere was Jerry Brown, who fell to Bill Clinton in California’s June 1992 primary by 9 points. A Santorum loss here would provide powerful evidence that the demographic barriers Romney has faced are weakening. It would also make Santorum a laughingstock.

If Santorum can clear these hurdles – wins in Louisiana, Wisconsin and Pennsylvania – he’ll probably be able to justify pushing on into May, when the calendar becomes more favorable for him. He’d be even further behind Romney in the delegate chase, but he’d still have a theoretical chance to deny his opponent 1,144. Anything short of the Louisiana/Wisconsin/Pennsylvania trifecta, though, will undermine the logic that’s keeping Santorum in the race.

But so far, the Texas governor seems to come out of the Haley Barbour school of leadership. Earlier this year, when the Sons of Confederate Veterans wanted Mississippi to issue a license plate commemorating Confederate hero and KKK founder Nathan Bedford Forrest, the governor refused to come out against the proposal, even after the NAACP asked him to. “I don’t go around denouncing people,” Barbour told reporters, although he was asked to denounce the proposal, not people. “That’s not going to happen. I don’t even denounce the news media.” At the time, the Mississippi governor was openly mulling his own presidential run, on an apparent campaign platform of Jim Crow racism being not “that bad,” in his memory, at least. Barbour announced he wasn’t running in April, after that trial balloon turned out to be made of lead.

To date Perry hasn’t taken a stand on the Sons of Confederate Veterans license plate. (Salon’s Justin Elliott ran down his quiet but savvy flirtation with his state’s Confederate forces in July.) Now Texas Rep. Sheila Jackson Lee is pressing Perry to oppose the license plate proposal, while the Sons of Confederate Veterans are demanding he support it, calling it a “litmus test” of Perry’s political courage and his willingness to support his state’s Confederate die-hards. It turns out I agree with the SCV on that point.

So what will Perry do? He’s doubling down on his birther idiocy, telling John Harwood that the president’s birth certificate “is a good issue to keep alive.” Expecting Perry to do the courageous thing is asking to be disappointed, but many people are watching.

I discussed Perry’s dilemma with Ed Schultz Monday night on MSNBC’s “The Ed Show.”

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Between all of this, Perry also made some news, using a speech to the Western Republican Leadership Conference to announce that he will soon propose a national flat tax. How Perry’s plan, which he said he will detail next Monday, is received may end up determining whether he moves back into contention in the GOP race or continues his slide toward irrelevance.

On one level, the move reeks of cynicism and desperation. Perry has been a candidate for more than two months, but hasn’t said anything about a flat tax before now. In that same time, he’s watched Cain overtake him in the polls while talking up his 9-9-9 tax plan — a call to throw out the existing tax code and replace it with flat income, sales, and business transaction rates. This has allowed Perry to see which aspects of Cain’s plan have gone over well with Republicans and which haven’t. For instance, the new national sales tax that Cain is proposing has caused him considerable grief; not surprisingly, Perry’s plan apparently won’t include one. It’s as if Perry has been using Cain as a flat tax stalking horse.

But that doesn’t necessarily mean this is a bad strategic move.

Cain’s recent surge in the polls reflects Republicans’ disappointment in Perry’s performance as a candidate and their reluctance to climb on board the Romney bandwagon. It also demonstrates just how popular the basic concept of a flat tax is. But Cain had a rough night at Tuesday’s debate, with his rivals piling on the 9-9-9 plan while he struggled to formulate a coherent defense of it (for example: “We are replacing the current tax code with orange”). It’s too soon to say for sure, but it may be that this marked a turning point for Cain’s candidacy, with Republicans who instinctively like him and the flat tax idea concluding that his plan is riddled with troubling imperfections and would make an easy target for Democrats — and that Cain himself is not nearly as strong a salesman as they initially thought.

If this is the case, it could mean that Cain’s days as the main alternative to Romney are numbered, and that there’s an opening for someone else to claim the role. It could also mean that there’s an opening for someone to present a more carefully-constructed flat tax plan, which seems to be what Perry is doing. Given the constant, almost comical upheaval that has marked the GOP race, there’s really no reason why Perry can’t gain back the ground he’s lost to Cain in the past few weeks, something that Steve Forbes, who built his 1996 GOP campaign around a 17 percent flat tax, predicted to the Daily Caller on Tuesday will happen:

“I’m elated by it,” Steve Forbes told me. “And I think Governor Perry will surge ahead of Herman Cain,” he said.

“Herman Cain gets credit of realizing the [current tax] code has to go [but] the virtue of what Governor Perry is doing is that he does not bring in a sales tax,” he said.

The challenge for Perry, if he is able to regain traction with his flat tax push, will be improving his own salesmanship. Exactly how he structures his plan remains to be seen, but the reality is that just about any flat tax proposal from a presidential candidate is going to have some significant flaws. He could (like Cain) faces charges that his plan is regressive. Or if he takes steps to guard against this (by imposing exempting incomes below a certain level from taxation or by maintaining certain tax deductions, for instance), his opponents could claim that the plan would blow a hole in the deficit.

Several previous presidential candidates have offered flat tax proposals, including Republicans Mike Huckabee, Dick Lugar, Phil Gramm and Forbes as well as Democrat Jerry Brown. The specifics of their plans all differed, but each received the same basic treatment Cain got from his fellow candidates on Tuesday night: Yeah, it’s a bold idea and it’s tempting, but…

So far, Perry’s debate performances suggest he’ll struggle to fend off that sort of attack. Which could be a problem because history suggests that it’s easy to get voters (especially Republican voters) excited about a flat tax, and just as easy to make them to go wobbly — something that Cain may be discovering now.