There's a theory in American politics that a protracted, hard-fought primary season is a thoroughly good thing for the party concerned. The eventual nominee will be more finely tuned to voters' concerns and better prepared for the real battle ahead, and the tough shots he can expect from his opponent in the general election campaign. Exhibit A for this argument is the Democrats' experience in 2008, when Barack Obama was undoubtedly improved as a candidate by his long struggle with Hillary Clinton. But don't mention the theory to Mitt Romney right now.

Super Tuesday, traditionally decisive, has come and gone. By the time you read this article, almost half of the 50 states will have held caucuses and primaries, but nothing has been resolved. Romney has had his ups and downs, and remains the frontrunner, with victories in six of the 10 Super Tuesday contests and more than twice as many convention delegates as his rivals combined.

In boxing terms, he has piled up a solid mid-fight points lead, and everyone expects him to prevail in the end. Alas, this year Republicans have brought in a proportional system to allocate delegates, in order to preclude an early knockout. This means he must keep slugging away until the end of the bout, or at least until his opponent – or rather opponents – realise that further resistance is futile and throw in the towel. But there is no sign of that happening. And with every week the fight drags on, the greater the damage to Mitt Romney.

His pluses and minuses are well known. Romney is competent and well versed in the economic issues that will decide the result in November. He has the party establishment lined up behind him, while Republican voters believe he is the candidate best equipped to defeat President Obama. On the other hand, he is not a natural campaigner. Try as he might, Romney often comes across as out of touch and elitist, while Tea Partiers, evangelical Christians and people who describe themselves as "very conservative" simply do not trust him.

These constituencies are most strongly represented in the south, where Romney has not won a single state. True, he has prevailed in Florida, and in Virginia last week. But the former is "southern" only in its northernmost part; the rest of Florida is a cross-section of America, heavily leavened by elderly Yankee sun-seekers who in their native habitat would overwhelmingly be Romney supporters.

As for Virginia, once the anchor of the old Confederacy, it has become a classic swing state, thanks to the explosion of the Democratic-leaning suburbs of Washington DC. Obama won the state comfortably in 2008, and is currently leading any Republican there in 2012. What is more, neither Rick Santorum nor Newt Gingrich, the candidates favoured by Tea Partiers and social conservatives, was on the ballot. Even so, Romney could win only 60 per cent of the vote in Virginia, with the remaining 40 per cent going to the libertarian Ron Paul, a niche candidate if ever there was one. In the two other southern contests that day, Gingrich won his home state of Georgia, while Santorum won Tennessee.

This week poor Mitt's water torture could continue. Yesterday, Santorum comfortably won Kansas by 51 per cent to Romney's 21; while in Wyoming, the latter won six delegates to his rival's three . On Tuesday, the contest moves on to the Deep South, as Mississippi and Alabama hold their primaries. Neither is Romney territory, but the latest polls are extremely close; if Gingrich and Santorum continue to split the evangelical and conservative vote, he could pull out a narrow victory in one (or conceivably both).

However, even a hitherto elusive success in the South could be a mixed blessing. Two defeats in his regional backyard may force Gingrich to withdraw, and create a straight fight between Romney and a single conservative opponent, Santorum. Romney almost certainly would still win – but at what price?

Herein lie the two big differences between the Democratic primary battle in 2008 and Romney's ordeal now. The marathon struggle with Hillary Clinton not only toughened Obama as a general election candidate; it was also a model of civility compared with the mud wrestling between Romney and his opponents, financed by record spending – almost all of it on negative advertising. As wife of one president and mother of another, Barbara Bush has had a ringside seat at four White House campaigns. Last week she called the one unfolding now "the worst I've ever seen in my life" – and few neutral Republicans would disagree.

The irony of these primaries in Mississippi and Alabama is that whoever does emerge as the Republican nominee will win them in a breeze in November. Republican voters may have their doubts about Romney. But his problems in the south are nothing compared with those of Obama – liberal, urban, northern and black to boot.

And there is another crucial distinction. Clinton-Obama was a battle for the political centre ground, but this Republican contest is a flight from the centre, a battle to prove who is most conservative. To Santorum, this battle is second nature. To Romney it is not. He is forced to take positions that do not come naturally. These may please (or, rather, appease) his audience in the primaries, but they will alienate centrist voters whose backing he needs to win in November.

The tougher he talks on abortion, the more women will decide Democrats are a safer bet. The harsher his language on immigration, the fewer Hispanics, the fastest growing minority, will be inclined to vote for him. Romney knows all this – you can read it on his face, if not in his language. Make it end, make it end. But it won't, not for weeks or months yet.