If I were a cautious, centre-left prime minister, Trump’s victory would have me worried | Andy Beckett

Whatever determinedly positive things centre-left leaders around the world have said about Donald Trump's victory in public, in private they must have greeted it with a shudder. Not just because of the dark and chaotic prospect of another Trump presidency, but because in many ways the defeated Kamala Harris is just like them. She is a hard worker, a patient reformer, a reasonably good communicator, an instinctive mover towards the ideological centre, a supposed antidote to rightwing populism, and yet also an incumbent, in an era when such perceived protectors of the status quo are widely despised.

Keir Starmer may have particular cause to worry. On her campaign website, Harris promised to "bring together" trade unions and business, "grow the economy" and increase both basic pay rates and employment. She said she had voted for legislation "creating hundreds of thousands of high-quality clean-energy jobs", and "ensuring America's energy security". She said she would "cut red tape" to "build more housing". She pledged "tough, smart solutions to secure the border ... and reform our broken immigration system." Above all, she presented her rightwing opponent as "cruel", "dangerous" and "unfit to lead".

All these policy ideas and political messages, and sometimes their precise language, could come from a Starmer speech or Labour press release. If they've been rejected by voters in the US at the first opportunity, could that also happen here?

Supporters and members of the Starmer government who want to believe that Harris's defeat is not cause for panic can point to the Conservatives' weakness compared with the Republicans. While the catastrophes of Trump's first presidency, such as his mishandling of Covid, appear to have been forgotten by many Americans, the Tories are weighed down by their terrible recent record in office, and are likely to be for years to come.

Britain and the US can also be very different politically. In the week that the notoriously reactionary Conservative membership nevertheless elected Kemi Badenoch as party leader, many Americans seem to have been put off by Harris's race and gender. Yet other contrasts between the countries are less reassuring. While the administration of which Harris is part has a strong economic record, Starmer's government is likely to bring only a modest improvement in growth and living standards, according to the official forecasts that accompanied last week's budget. If many voters did not notice, or refused to give Harris credit for, the boom under her and Joe Biden, what chance is there that Starmer's probably smaller economic successes will be electorally rewarded?

This apparent breakdown in the relationship between a government's achievements and its popularity poses a profound threat to centre-left politics. For decades, centrists have assumed that "what counts is what works", as Tony Blair put it. As its name implies, centre-left politics is about compromise and alliances, which are meant to make steady, measurable progress on concrete issues. Yet it appears that more and more voters prefer the dogmas, tribalism, symbolic gestures and fantasy policies of rightwing populism. This dramatic, accelerated, often more short-term politics comes across better on digital media. It also expresses many voters' anger about the present and anxiety about the future – or their desire to ignore looming disasters such as the climate crisis for as long as possible.

In the two previous periods when western democracies were consumed by doomy thoughts, the 1930s and the 1970s, many centre-left governments also struggled and were sometimes replaced by authoritarian rightwing populists. At prime minister's questions this week, hours after Trump's election, there was a new mood, which could not just be attributed to the fact that Badenoch was making her debut. She beamed with satisfaction at Trump's victory, and woundingly remarked that Labour's budget had been "cut and paste Bidenomics". Meanwhile, Starmer gave unconvincing assurances that Anglo-American relations would continue as normal.

In these exchanges was possibly the beginning of a political shift: towards a situation where his government, while still theoretically dominant at Westminster because of its majority, in fact loses the ideological initiative and becomes isolated, even beleaguered.

We're not there yet. Despite her aggression, Badenoch is not a commanding public performer and may never be one, given her tendency to bluff and her party's lack of credibility and fresh ideas. Labour also has time on its side. By our next election, Trump's final, four-year term may be over – and may also have demonstrated, as he did last time, that populists are better at electioneering than governing.

It's possible that his latest victory will be the Republican equivalent of the Tory win in 2019: achieved by making impossible promises in circumstances that favour the right to a greater than usual extent, with Biden's infirmity and a rare spike in US inflation analogous to the huge but fleeting Conservative opportunity created by the vote for Brexit.

Yet simply waiting for Trump and other populists to fail in office again would be a slow and uninspiring strategy for the centre left: an acceptance that change can only come after further, possibly terminal, social and environmental damage. Instead, the centre left could make a better case, whether in government or opposition, by addressing inequality with more urgency, as Biden did before beating Trump in 2020, having incorporated ideas from Bernie Sanders' insurgent campaigns to become the Democratic candidate.

We live in a different world to the one that formed the modern centre left. Unless it becomes more aggressive and more class-conscious – effectively, more populist – it will continue to rule only occasionally and with modest success. The rest of the time, the radical right will run riot.