Many strange things have happened in America, in this young age of Obama. But none stranger, surely, than that the de facto leadership of the Republican Party has passed into the hands of a right-wing talk radio host.

The moment of that passage came on Saturday 28 February, when Rush Limbaugh was the closing speaker at CPAC, the annual Conservative Political Action Conference here in Washington, normally a festival for Republican true believers. As I wrote in this space last week, the arriving delegates looked like stragglers from Napoleon's army in the 1812 retreat from Moscow, shellshocked still by their crushing election defeat. But when they left they were walking on air – or rather, the reverberating echoes of Limbaugh's address, in which he reaffirmed conservative values and stated that he wanted the new President to fail.

For many, it was an unsettling moment: not because of the black shirt he was wearing (Limbaugh's bloviating has always had a whiff of Il Duce), or because John McCain, the party's defeated White House candidate, had chosen not to attend. What made it unsettling was that politics had not merely fused with entertainment. It had surrendered to entertainment.

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In some ways, this new hour of Rush is no surprise. Ever since 1987, when federal regulators dropped the "fairness doctrine" that required stations to balance competing points of view, conservative talk radio has been the trumpet section of the Republican orchestra. And for almost as long, its acknowledged leader has been Limbaugh, heard on 600 radio stations and commanding a daily audience of 20 million or more.

In these 20-odd years, conservative radio has come a long way. By coincidence, on the very same day as Limbaugh was propelling the genre to new fame or infamy at CPAC, Paul Harvey died in Arizona at the age of 90. In many respects, Harvey was Limbaugh's forerunner. His show ran a mere 15 minutes, compared with Limbaugh's three-hour blast every weekday. But he offered his own mix of news and and conservative comment, with similar stratospheric ratings. In his heyday, in the Sixties, Seventies and Eighties, Harvey had 20 million listeners. But he was cheerleading not so much for Republicans as for the American way, at the zenith of the American century. Never, one suspects, would he have lent himself to something like CPAC.

Limbaugh, however, is unabashedly, exultantly, partisan. Indeed, when Bill Clinton recaptured the presidency for the Democrats, Ronald Reagan hailed Limbaugh as "the number one voice for conservatism". And Rush did not disappoint, stoking the voter anger that fuelled the Republican takeover of Congress in 1994, two years after Clinton entered the White House.

Inevitably, talk radio thrives in opposition. No one had the hosts spluttering with indignation quite like Clinton, with his slippery ways and the wife they likened to Madame Mao. George W Bush, on the other hand, ended up as much an embarrassment for Limbaugh and his kind as he was a liability for his party. But now the Democrats are back, and it's open season.

Limbaugh probably went too far by expressing the hope that Obama would fail. But that hasn't stopped his listenership soaring since the election, or his rotund frame from filling the void at the summit of the party.

Such power vacuums are normal here, but rarely as obvious as now. Unlike Britain, America only has a real leader of the opposition for the brief period between nomination of a presidential candidate and the election. The highest-ranking Republicans in Washington are now Mitch McConnell and John Boehner, minority leaders in the Senate and House respectively. Neither is a galvanising figure.

Bobby Jindal, the 36-year-old Governor of Louisiana who is touted as a future White House candidate, had his chance when he was selected to deliver the Republican response to Obama's address to Congress, a couple of days before CPAC. But Jindal blew it, with a performance that made even Republicans cringe. And then there's Michael Steele, the new chairman of the Republican National Committee (RNC), who has a claim to leadership in his capacity as head of the national organisation. But Steele, far from taking matters in hand, grovelled to Limbaugh.

One of the trustiest ploys in US politics is to demonise a celebrity figure of the opposing party. The Republicans did so with Michael Moore, the documentary maker, whom they portrayed as a crazy liberal pulling the Democrats' strings. Now the Democrats are setting up Limbaugh as a similar target. The day after CPAC, Rahm Emanuel, Obama's chief of staff, proclaimed the radio host as the "intellectual force and energy of the Republican Party". The hapless Steele rose to the bait. Nonsense, he said. Limbaugh was merely "an entertainer". But when the entertainer hit back by saying that the RNC chairman had "got off to a shaky start", Steele kowtowed. He had, he said, been "inarticulate" – in other words: sorry sir, I won't get out of line again. But it may already be too late. Limbaugh's supporters are outraged. At least one committee member is calling for Steele's resignation.

Such are the risks of crossing Rush Limbaugh. He may not be leader of the party, but right now he is its most powerful figure. For what other Republican has such an audience, not to mention communications skills rivalling those of Reagan himself.

But these same attributes make him a gift for Democrats. The conservative base adores Limbaugh, but not many others do – certainly not the self-described independent voters who decide elections, and who currently dislike him by a three-to-one margin. The louder Rush roars, the quicker the funds flow into the Democratic Party coffers.

However, there are dangers for Democrats, too. "Always fight up, never fight down," was Reagan's advice for presidents: go after worthy targets, don't get dragged down into the political gutter. The Obama White House may be winning the mud-wrestling, but at the risk of betraying the lofty principles on which they won the election. And one day, even Obama's lustre will tarnish. The Bush recession will turn into the Obama recession (or depression), and even independents may think Rush has a point.