It is exactly 16 years since Labour last won a general election – an anniversary the party does not celebrate. On 5 May 2005, Tony Blair beat Michael Howard. The Conservatives responded by listening (with varying degrees of enthusiasm) to David Cameron’s gospel of brand decontamination. By electing Cameron as Howard’s successor, the Tories showed they had understood what Theresa May first told them in a party conference speech back in 2002 – that they were seen as the nasty party. Cameron’s modernisation project was neither thorough nor profound, but it worked well enough to require a shift in electoral strategy from Labour. The challenge goes unmet to this day. Labour still campaigns against the “nasty party” because it is the one they can beat. There have been many lines of attack against a parade of Conservative leaders, but the underlying message is consistent: same old Tories. That is the background music when Keir Starmer challenges Boris Johnson over mystery donations that funded the Downing Street makeover. In the Commons last week, Starmer called the prime minister “Major Sleaze”. The allusion reaches back to the Tories’ mid-90s slide into rancid disrepute and out of office. It is right for the opposition to hold the government to account over corruption, but there is a nostalgic inflection in the way Labour hammers at the word “sleaze” – and it speaks of insecurity. Starmer is trying to ignite flames of public indignation by blowing on 25-year-old embers. Local and devolved elections on Thursday might show that the message is getting through, but Labour MPs and activists who have been knocking on doors in marginal areas are not optimistic. They say the rotten smell has reached the public nose but is not overpowering. It does not beat the prospect of restored local and national esteem that many former Labour voters heard two years ago in Johnson’s invitation to “get Brexit done”, and which resonates even though the UK has left the EU. It doesn’t help that Labour is also battling a perception in its former heartlands that the party has been captured by snooty metropolitans, far-left fanatics or both. That cohort of angry ex-loyalists does not feature on Labour’s old mental map of Conservative voters. The standard model depicts the Tories as a machine for servicing the rich at the expense of the needy; hardwired for cruelty. When Tories show a compassionate streak, the automatic left response is to dismiss it as a cynical feint. When Conservative governments flirt with egalitarian policy – as when Johnson speaks of “levelling up” – the left predicts failure through ideological rigidity. Callous free-market capitalism is too deeply ingrained, it is said, and the Tory imagination is closed to social justice. There is enough truth in that picture for the opposition to avoid dwelling on the gaps. But it is not the whole truth, which is why Britain keeps electing Conservative prime ministers. The left can account for those results without enduring the psychological trauma of admitting that good people have sincere reasons to reject a Labour leader. Aggressive media bias is the most reassuring explanation. That, too, is true enough. It satisfies anyone who would rather not interrogate the causes of opposition failure for too long. You can point to rightwing tabloid hysteria and call off the search. But blaming the media slips easily into blaming the electorate for being too stupid to see past the headlines. Labour does not endear itself to voters by treating the ballot paper as an exam and tutting when people give the wrong answer. A strategy based on telling voters to look again, look harder, because they missed the point last time is doomed, as anyone involved in the campaign for a second Brexit referendum can attest. A variant of the same problem is the habit of pointing at Tory prime ministers and claiming their “mask is slipping”. Starmer used that line against Johnson over nurses’ pay in March. It is what Ed Miliband used to say of Cameron. The mask metaphor is how opposition leaders say “I told you so” to voters. (Never an attractive look.) Calling out the slippage implies that Labour, in its superior wisdom, knows truths that lie hidden from the credulous electorate. A newer source of Labour denial is to exaggerate Johnson’s charisma. The prime minister is imbued with ineffable mesmerism that hypnotises voters into forgetting their horror of the Tory brand. Once again, true enough. Without the extra spark of the “Boris” persona, the 2019 election might have been closer. But that doesn’t explain how Conservatives won the Tees Valley and West Midlands mayoralties in 2017, when May fought a weird, self-sabotaging campaign reminding voters of Tory nastiness more efficiently than Labour could. Little noticed amid the fallout from May’s implosion in that election were the increased Conservative vote shares in northern English seats Johnson would go on to win two years later. The shift in Britain’s electoral geography predates the “Boris effect”. Johnson’s shtick will wear thin eventually, and that will be an opportunity for Labour. But lost voters will not snap out of a trance, wonder by what witchcraft they were seduced into being Tories, and pick up their old ancestral allegiance. In over a decade of opposition, Labour has offered many policies that it thinks voters should like, buttressed with solid reasons to reject the Tories. It hasn’t worked. One thing no leader has attempted is a candid, humble explanation for why the party keeps losing, complete with a credible display of understanding why voters switched sides. It doesn’t come easily. The left is conditioned to doubt that any decent human being could be a Tory. That is the barrier to empathy across which no minds are changed. The alternative is to keep running the same old campaign against the same old Tories, and to keep on losing because Britain doesn’t have the same old politics, or the same old voters. Rafael Behr is a Guardian columnist