They say one in four of the world's people will have a vote in an election in 2012, but no contest will get more attention than the presidential one in the US – if only for its entertainment value. So far the Republican primary has spoiled us, from Rick Perry's "oops" to corporate asset-stripper Mitt Romney's admission that he liked firing people, delivered just before he was snapped apparently receiving a sit-down shoe-shine from an underling – not a good look for a would-be man of the people. (It later emerged that he was in fact being 'wanded' in an airport security check.) En route we've had Rick Santorum insisting that he does not equate homosexuality with bestiality – or, as he memorably phrased it, "man on dog" – and that when he had appeared to make a disobliging reference to black people, he had in fact been speaking of "blah" people.

All amusing enough in its own way, but no match for this time four years ago, when the race between Barack Obama and Hillary Clinton captured a rapt and global audience. The 2008 contest and its result achieved a remarkable turnaround in the US's standing overseas. The Pew survey found that while just over 50% of Britons, for example, had a favourable view of the US in the final Bush years, the figure had leapt close to 70% by the time Obama was in the White House.

Yet now the numbers are slipping again. I have my own unscientific indicator to go on too. A few years back I published a book calling for Britain to learn from America's founding ideal, to reshape our own creaking political machinery on the lines of the US constitution, with its separation of powers and guaranteed rights. Soon after publication, events conspired to make the US a hard sell. Whether it was the Monica Lewinsky-related impeachment of Bill Clinton, the Florida fiasco in which Al Gore seemed to lose an election he'd won or the entire Bush presidency, I was regularly confronted with the original subtitle of my book – How Britain Can Live the American Dream – and mockingly asked, "It's all looking like a bit of a nightmare now, isn't it?".

I'm hearing that again, as non-Americans watch not just the bizarre Republican presidential field but the paralysis of a US political system that has rendered an elected president apparently incapable of doing almost anything. The final straw came last August, when the US saw its credit rating downgraded after coming close to a default – all because Congress refused to raise the country's debt ceiling. Surely now, people wondered, I had given up my youthful enthusiasm for a set-up that could result in such madness?

Well, no, I haven't. I still admire a system in which election is the usual method for allocating public positions, including the head of state; whose second chamber is elected rather than appointed; which ensures serious power exists at local, town hall level; which locates sovereignty in the people rather than in an abstraction, such as our "crown in parliament"; and which sets down the rules and rights of national life in a written constitution that serves as a kind of owner's manual available to every citizen. All that I still admire.

But I confess the constipation embodied by the US Congress, the constant gridlock, has made me despair. A check on the executive is one thing; a triple-locked pair of handcuffs on the president's wrists, restraining him and his party from even, say, extending unemployment benefits to the needy, is quite different.

And yet this is not some inherent flaw in the US system, an outcome logically entailed by the founders' design and therefore unavoidable. On the contrary, it is the result of an abuse of the system, a consequence specifically of the march rightward of the Republican party.

Take the debt ceiling row. Congress never used to have a problem with that: the vote to raise the limit was always routine, nodded through 87 times since 1945, no matter which party was in charge. But today's Republicans seized on the chance to put a gun to the head of the US economy. Either the president caved to their demands or they were ready to see the country default. No wonder the credit agency Standard & Poor's declared as it took away America's triple-A rating that "the effectiveness, stability, and predictability of American policymaking and political institutions have weakened".

To be sure, that ability to hold the country to ransom was always there, buried within the rulebook. But convention and a shared assumption that no party would act blatantly against the national interest ensured those potential weapons lay dormant. Starting in the 1990s, under then-speaker Newt Gingrich, the Republicans cast aside those conventions, dismissing them as the cosy practices of Washington insiders, an offence against ideological purity. More important now is their Tea Party pledge to vote against any tax rise or new borrowing, no matter how damaging the impact.

So they threaten filibuster against any important Democratic measure and every presidential appointment, a trick that can only be foiled with 60 out of 100 Senate votes. That way the Republican minority exercises a veto over the Democratic majority, even if the result is paralysis in the face of economic crisis and hundreds of crucial government posts left empty. Again, the possibility of minority rule may have been there before. But it has taken the shift in today's Republican mindset to realise it.

And what is that shift? It is towards an anti-government fervour that recalls the militia movement of the 1990s, convinced that every Washington move – even a plan to expand healthcare – is motivated by wickedness and constitutes a step towards tyranny. In that context, any action to thwart the government beast is justified. Such ideas were always around on the lunatic fringe, but they have entered and now dominate the Republican mainstream. Today's presidential candidates must bow to them. In the words of Mike Lofgren, a Republican congressional staffer who recently quit after nearly 30 years' service, today's Republicans are less like a traditional political party and "more like an apocalyptic cult".

There is a bitter irony here, that the party that insists it is conservative and patriotic now threatens the centuries-old political system that lies at the core of the US's national identity. The ideal remains true, but it is being warped almost to breaking point by the very people who claim to be its loudest defenders.

Twitter: @j_freedland

• This article was amended on 20 January 2011 to clarify that Mitt Romney was not photographed having a shoe shine.