The Government repeatedly tried to paint the Conservatives as a party of the extreme Right in the European elections and in the Kensington and Chelsea last week.
But the tactic had no impact at all as the Tories romped home in the Euro elections and Michael Portillo increased the party's share of the vote in the by-election.
Worse still, Labour's own private polling has found that voters link "extremism" with the "Loony Left" rather than the "Raving Right".
The polling, conducted after the party conferences, was done after speeches by Tony Blair, Gordon Brown and Alan Milburn, all of which claimed the Tories were prisoners of extremism.
Labour ran its own "X-files" campaign this summer in the run-up to the European Parliament elections, with a succession of ministers claiming the Tories were extreme on health, education and welfare policy.
But the media refused to pick up the issue and concentrated instead on the two parties' differences over the single currency. Rather than finding William Hague extreme, the voters appeared to like his policy of "Save the Pound".
During the Kensington and Chelsea by-election, Labour produced leaflets and posters attacking Mr Portillo personally, accusing him of being arrogant and extreme.
Insiders at Labour's Millbank HQ are now worried that the tactic has backfired as badly as the Conservatives' own New Labour New Danger "Demon Eyes" campaign at the last general election.
"It has bombed with the voters," one senior Labour source said. "We've been making the mistake the Tories made with Demon Eyes in 1997. Banging on about how extreme they are is not registering with the public.
"It's all too abstract. Instead of talking about extremism, we should have been pointing out the benefits of joining the euro - low interest rates and mortgage rates, for example."
Another senior Millbank insider agreed that campaigning tactics were being overhauled in the light of the European and Kensington and Chelsea results.
"It's no good just calling them extreme," he said. "What we need to do is to is pin down the policies that are extreme, such as more private sector involvement in the health service. We were trying to do that in the latter stages of Kensington and Chelsea.
"We want to refine the tactic, not dump it altogether. The Tories may try to come back to the centre ahead of the next election. We've got to be prepared for that."
Downing Street and Millbank are convinced the Tories' Common Sense Revolution has shifted them to the Right, but there are worries about which line of attack to use.
In the summer, Labour researchers produced an X-file on Ann Widdecombe, the shadow Home Secretary, claiming that she was in favour of handcuffing pregnant prisoners and forcing the public to pay to see their GP.
"This is not an X-file. It is execrable," a Tory spokesman said at the time.
Just as worrying for Labour were signs that the controver-sies surrounding Lord Archer of Weston-super-Mare and Michael Ashcroft had little or no impact on voting patterns in Kensington and Chelsea. The party's main by-election poster showed Mr Portillo with Lord Archer early in the campaign. Its leaflets described the former defence secretary as "uncaring, untrustworthy, unelectable and arrogant".Reuse content