John Rentoul: Only Johnson can hold back the Tories

Something has happened in British politics. Our ComRes poll today suggests that much of Labour's core vote has been surgically removed and transplanted to the Liberal Democrats. This is little to do with the positive attraction of Nick Clegg, or even of Vince Cable, who will this week reprise his role as part stand-up, part human, part economic clairvoyant while Clegg is on paternity leave.

It has everything to do with Gordon Brown's assets turning as toxic as the bad debts that have brought the banks low. He was saved from the pit of oblivion when the banks went bust in the autumn because he not only seemed to know what he was doing but did do many of the right things. He was praised taking stakes in two of the largest bank groups and for pumping money and credit into the economy.

But getting a couple of things right is to no avail when greater errors come home to roost, as they have this month. As with all historic events – the Reformation, the French Revolution, the slump in Gordon Brown's poll rating – there are short-term and long-term causes.

The immediate cause is the popular fury over bonuses. And who is the seditious leftie rabble-rouser that whipped it up? The President of the United States. It was Barack Obama's populist announcement 11 days ago of a $500,000 cap on pay in bailed-out banks that caused the pot to boil over. Why can't we do that here? was the first reaction. And the second reaction, when we recalled that Obama was the new guy and that Brown had bailed out British banks with our taxes months ago, was: Why didn't the Prime Minister do it in October?

Alongside banker rage is jobs angst, with Brown's empty promise of "British jobs for British workers" infuriating the blue-collar base even more than it offended the liberal middle class when he first said it.

Then there are the long-term causes of Brown's downgrading identified by ComRes. He was the architect, along with Ed Balls, now his preferred successor, of the new system of City regulation brought in 12 years ago. Again, triumph and disaster were conjoined twins. Just as Brown did the right thing last year in part-nationalising the banks, but at the same time failed to get a grip on excessive pay, so he did the right thing in making the Bank of England independent, but at the same time took away its responsibility for regulating the banks. The Financial Services Authority, whose deputy chairman Sir James Crosby resigned last week, has been a weak regulator.

So that is that. Not just at the level of perceptions – Brown ran the economy for 11 years and takes the blame when it goes belly-up in year 12 – but at the level of reality. He got a lot right but bears policy responsibility for two crucial aspects of the present crisis: regulatory failure and excess bonuses.

Yet the absence of a gravitational pull from Planet Cameron is almost as striking as the eclipse of Brown. Our poll has the Conservatives down one percentage point since last month. Labour's loss has been the Lib Dems' gain, as the default option for Labour supporters losing faith with the Government but not convinced that Cameron has the answers. The Lib Dems can pull in the protest votes, almost regardless of who their leader is or what their policies are, as has been illustrated by their recent history. Their poll ratings have fluctuated as the left-overs from the two main parties, independently of whether their leader was sober or a stand-in.

If Labour's decline is not yet good news for the Conservatives, it surely marks the last oscillation of Brown's descent to defeat. The defection to the Tories of David Freud, Labour's welfare adviser, is a pointer to the way that power is shifting.

I suspect that Labour has one last chance to avoid a crushing loss at the next election, which is why it is interesting that Alan Johnson, the Health Secretary, has been out and about last week. In the first of two interesting interviews, he showed how to take on Cameron: by going along with the public view of him as "likeable", but by suggesting that his party wasn't. In the second, he showed that he was a modernising reformer, but going with the grain of public-sector attitudes. "The task we set ourselves was to change the language," he said. So the NHS reforms have continued, but without some of the management-speak or confrontational rhetoric deployed by the Blairites.

The result, he reminded us without saying so, is that the NHS has ceased to be a negative for the Government. All the fuss about trusts in deficit and doctors' pay has subsided. The corollary is that the positives have attracted little attention too: before Christmas the 18-week maximum wait was achieved ahead of schedule in a healthcare system in which people used to have to wait 18 months. But better to have positives under the stones than more horror stories.

Yes, I know that we have been here before, in 2006, when Johnson was the candidate we Blairites thought would be a better prime minister than Brown. That proposition was never tested, and when it came to it Johnson could not even win election as deputy Labour leader. And yes, I know that Johnson himself said, a few months after Brown took over: "I don't think I would have been good enough, frankly. I don't think I've got the capabilities."

But times change, and admissions of limitations can seem more attractive as the clouds darken and others, in all parties, are found wanting. I am not saying that the Cabinet will mount a putsch against Brown and install the collegiate and witty Johnson as the "brace position" prime minister to minimise Labour's certain defeat.

One cabinet minister recently ruled out another spasm of leadership speculation, telling me that it was the "settled will" of the party that Brown would lead it into the election. I suspect that there is truth in that: that it is the settled will of the party that it will fight on with as much dignity as it can muster, but it is not actually going to do anything that might help save it from the impending crash.