Michael Steele, the new chairman of the Republican party, is surely setting some sort of land speed record for public-relations disaster. Elected on 30 January, he barely lasted a month before the first calls came for his resignation. The month in question included various oddities on his part, most notably his disastrous attempt to show he was a bigger dog on the Republican porch than radio fat-mouth Rush Limbaugh, which ended with Steele in full retreat, tail between his legs.

The calamity proved that Limbaugh, in Republican and conservative circles, is not to be tampered with. Everyone has observed that. But few have taken the lesson one step further and asked what, in turn, is proved by Limbaugh's preeminence within the party. Within the answer to that question lies Steele's biggest task as chairman, but it's a task that, for implacable structural reasons, he'll find impossible to take on. If you've been wondering why the Republican party is such a train wreck right now, this is why.

Limbaugh is a dominant figure because the Grand Old Party is no longer a political party in the usual American sense. It is an ideological faction. In America, as you know, we've had basically a two-party system for most of our history. In parliamentary systems, small ideologically driven groups tend to form political parties, win a few seats, and make coalitions with larger parties.

In America, it doesn't work like that. Our small ideologically driven groups have chiefly located themselves within the two big parties and fought for power internally. For instance, the Democratic party has, since Franklin Roosevelt's time, been an amalgam of clashing interests. Notably, FDR's Democratic party included northern liberals and southern racists (many of whom were liberal on economic and redistributionist questions as long as the redistributing was limited to white people). By the early 1960s, though, the tension became too great and the Democrats made choices - good and courageous choices - that forced the racists to leave.

Meanwhile, from the mid-1950s, a conservative rump group decided to "burrow from within" and work inside the Republican party to take it over. The GOP of the 1950s, led by Dwight Eisenhower, was quite middle of the road by today's standards, and conservatives held Ike in contempt.

Well, to make a really long story really short, they succeeded. A cohort of moderates remained within the GOP through the early 1990s. Today? There are 41 Republicans in the Senate and 178 in the House of Representatives. Perhaps four of the former and 10 or 12 of the latter can be called moderate. The rest are committed conservatives.

This is insanely out of balance for an American political party. You look at the Democrats, and they aren't uniformly liberal in the way the Republicans are uniformly conservative. Of the 58 Democratic senators, nearly 20 are genuinely moderate. This exasperates liberals and will get under President Obama's skin. But historically speaking, it is as it should be. American political parties are supposed to be big and diverse.

But today's GOP consists, with those few exceptions, only of its conservative faction. Conservatives have been disciplined and strategic and have poured billions into political infrastructure-building over the years. When things were going their way, they didn't have to worry about the lack of moderates. But now, things aren't going their way. Unless Obama really, really blows it, the GOP is going to be the minority party for quite some time. It's easy to see now why Limbaugh has such power, no? If the GOP had a moderate wing, he wouldn't.

Any sane person can grasp, then, that Steele should revive a moderate wing. But the conservatives will not permit it. Co-operation with the president is capitulation, and any vote or utterance that admits even the most modest role for government is socialism.

Steele himself seems to have no interest in changing direction. Incredibly, he said he would not rule out primary challenges from the right to moderate GOP legislators who support Obama on key legislation. One of the moderates, Arlen Specter of Pennsylvania, is up for re-election in 2010. A rightwinger who nearly beat him in a primary the last time just vowed to run again, and he will have support from conservative money sources. What will Steele do?

The likely result of a primary win by the conservative challenger, of course, is that the Democrats will pick up the seat, because the challenger, Pat Toomey, may be too rightwing to win in Pennsylvania. And if the Democrats get that 60th Senate seat in 2010 (60 is a filibuster-proof majority), the Republicans will really have no plan except to hope Obama fails.

Mind you, I'm hardly upset about any of this. I think it's all pretty wonderful. Certainly the selection of Steele, who is African American, played against type. He was supposed to be a breath of fresh air. So far the air is awfully hot and stale.

• Michael Tomasky is editor of Guardian America. Read his blog at theguardian.com/commentisfree/michaeltomasky