Chinese terrorists are streaming across the border. Barack Obama is a violent socialist. Mexico has been launching military attacks against America. God has endorsed Mike Huckabee. Spend a week with Republicans in South Carolina and you will hear the most incredible things. That a small minority in any group might say crazy things is not surprising; it is when the majority don't dismiss them as crazy that you start to worry. At first it sounds as though most of them are living in a state of suspended reality. But with time you realise that they have simply been marinating in the bellicose polemics of talk radio and rightwing news anchors for far too long. Their reality is specific and bespoke.

Having warped their understanding of how the world works to suit their ideology, they now have the terrible burden of having to live in it. On the whole, these are personally affable and politically angry people. The targets of their rage are clear: Hillary Clinton, the liberal media, illegal immigrants, Muslims, taxes, the government and nationalised healthcare all take their turns in the crosshairs.

But the source of their rage is a mystery. In George Bush, Conservatives have had almost everything they wanted. Tax cuts, war and conservative supreme court justices have all been forthcoming. For much of the time he has been in the White House the Republicans have controlled both houses of Congress too. To the faithful, that the economy is nosediving, the war has been judged a failure and the president's approval ratings scrape historic lows are tiresome details. Since they only have themselves to blame, they simply change the subject and hope no one will notice.

When Republican presidential hopeful Mitt Romney declares "Washington is broken" before a cheering crowd in Bluffton, you have to wonder who they think broke it. Romney went on to say, with a straight face, that he drew his inspiration from "Ronald Reagan, George Herbert Walker Bush, Nancy Reagan and Barbara Bush". When a leading presidential contender says he is enthused by the president's mother but won't mention the president himself, it becomes clear to what extent those who wish to be head of state must first occupy a state of denial.

The fact that the Republicans would rather pretend the last seven years didn't happen than deal with their consequences lies at the root of the agonising slugfest that is their primary race. Six elections into the nominating process, not one of their candidates can claim the consistent support of more than 25% of their voters.

At present, their primary battle remains a four-horse race. The trouble is the horses keep changing and one of them isn't even out of the gate yet. Romney, John McCain and Huckabee have all had their moment in the sun over the past few weeks only to be eclipsed by defeat. Huckabee has won one race; McCain two and Romney three (although two of those were barely contested). Meanwhile, Rudy Giuliani, once a national favourite, has yet to hit double digits and is waiting to kick off his campaign in Florida, which votes next week.

Each candidate has proven his ability to appeal to a specific segment of the Republican base or a particular state, but has largely struggled to gain traction beyond those local constituencies. Take Romney. On Saturday, he won Nevada with a barnstorming 51% in the afternoon only to finish the evening fourth in South Carolina (where until recently he had spent a lot of time and energy) with just 15%. The two results were linked.

He owed his huge margin of victory in Nevada in no small part to Mormons, who comprise just 7% of the state's population but made up 26% of the Republican caucus vote. According to exit polls, 95% of them voted for Romney, who is also a Mormon. In South Carolina, on the other hand, many wouldn't vote for him because he was a Mormon - white evangelicals make up more than half the Republican base there and only 11% backed him.

If they want someone who can take Ohio, Nevada, Florida and Pennsylvania - key swing states - in the presidential election in November, Republicans will need a candidate with a far broader and consistent national appeal than that. McCain's victory in South Carolina on Saturday suggests that might be him. He has now won in the libertarian north-east of New Hampshire and the socially conservative south. The trouble is that the Republicans don't really like him - he is, among other things, insufficiently pious and xenophobic.

"If either of these two guys get the nomination, it's going to destroy the Republican party," said the conservative talk-show host Rush Limbaugh referring to McCain and Huckabee. "It's going to change it forever, be the end of it. A lot of people aren't going to vote."

What we are witnessing is the unravelling of the alliance of evangelicals, southerners, white men, neocons, corporations and the wealthy that elected Bush. In truth, it was only ever a coalition of the convenient - and not a particularly brilliant one at that. Bush was a crude majoritarian. His aim was only ever to win 51% of the vote. Unlike Reagan, who managed to remould the electoral landscape by expanding the appeal of his party, there is no such thing as a Bush Democrat.

Indeed, the Bush coalition was not big enough to win the first election without the help of the supreme court and would not have scraped through the second election without a war. Now, in the absence of a candidate they can agree on and extenuating circumstances to push them over the edge, the various factions that helped him to power are at loggerheads. Bush has bequeathed the same legacy to his party as he has to the nation - division.

The Democrats' propensity to gloat at this cruel irony is understandable. It is also a mistake. True, while Republicans bemoan their paltry choice, turnout for Democratic primaries and caucuses has set new records and revealed rarely seen levels of enthusiasm. Judging by those taking part, the Democratic base is expanding and rejuvenating itself.

But it is no less true that the Democratic primaries have raised little in the way of political debate. Where a vigorous discussion about how to end the war, rebuild the economy or extend healthcare should be, they have instead indulged themselves in a beauty contest that is simultaneously entertaining, engaging and empty. It has been a battle of nouns - "experience" versus "change" - but not verbs. Quite what they would actually do with this experience or what they would change is not entirely clear.

The mayhem in the Republican party primaries is being driven by differences in policy; the order among Democrats has been created by an obsession with personality. Whoever wins the Republican nomination will have been tested; whoever becomes the Democratic candidate will have been crowned. The Republicans may be in desperate need of consensus; but the Democrats are in dire need of content.

g.younge@theguardian.com