Coming just days before millions go to the polls, Zia Yusuf’s announcement that a Reform government would “prioritise” the siting of migrant detention centres in areas with Green MPs or councils was certainly eye-catching.

“That means areas like right here in Brighton,” Reform’s shadow home secretary said with barely concealed relish in a video in which he paced the beachfront at the constituency that elected Britain’s first Green MP.

The policy was accompanied by the launch of a webpage where curious voters can enter their postcode to “check” the polls and see if their area was likely to be the site of a detention centre. Inputting E8 1EA – the postcode of Hackney town hall, where the Greens are this week tipped to win council elections – brings up a red box with an exclamation sign and the warning: “Yes – on the list. Your area will be prioritised to receive a detention centre under this policy. Stand with Reform to change that.”

Cue condemnation from Reform’s opponents on the left and right – the Greens and Labour described the policy as “disgusting” and “grotesque”, while the Conservatives dismissed it as “not a serious policy” and one “made up on the spot for a social media video”. Imran Hussain, director of external affairs at Refugee Council, described it as “unworkable and profoundly un-British”.

YouGov polling released on Tuesday indicated that 45% of more than 4,000 adults polled on the same day did not believe it was acceptable for a government to base decisions that affect individual constituencies on which party voters supported at a general election.

Even among Reform’s own voters, 37% believed such decisions were unacceptable, with 34% believing it was acceptable to do so.

So what are Reform playing at? At one level, the simple need to garner attention on social media was clearly a factor. By Tuesday, the video in Brighton had garnered 3.7m views on the X account of Yusuf who, like Green leader Zack Polanski, is without the relative benefits of having a parliamentary podium.

But a broader strategy of sorts also appears to lie behind the policy, which appears to have been largely cooked up in Yusuf’s own office, a product of a supposedly new party that Nigel Farage has characterised as less the “one man band” of old.

As one party insider put it: “Zia’s office moves in marvellous and mysterious ways.”

Above all is the desire of Reform to establish itself and the Greens as the two real choices in front of the electorate this week, particularly in English council elections.

“It’s clear that the failed uniparty era is over and there is a battle for the soul of our country between Reform and the Greens,” said Yusuf, who has previously repeatedly – albeit without luck – challenged Polanski to a live, head to head debate.

The primary audience for the policy is also Reform’s base away from areas where the Greens are expected to make gains, such as one-time Labour strongholds in London and other cities.

“Reform are a very modern political party, which farms outrage and wants people to be angry, so in a low turnout election – as local elections are – this is about ensuring that their voters continue to have something to feel strongly about,” said John McTernan, a former political adviser to Tony Blair.

“Reform are genuinely an authoritarian party and they say that they want to deport tens of thousands of people because they really want to do it. This new policy is the rhetorical flourish to get people talking about that policy.”

Reform’s core deportation policy was outlined last August when the party unveiled its “Operation Restoring Justice” document, in which it pledged to deport hundreds of thousands of asylum seekers, pay despotic regimes such as the Taliban to take them back, and rip up the UK’s postwar human rights commitments. A five-year “emergency programme” would identify, detain and deport illegal immigrants.

Less noticed this week was how Yusuf’s new announcement marked a pivot from that original document. No mention was made of Hackney, Lambeth or Brighton on that occasion. Instead, the party said that Secure Immigration Removal Centres (SIRCs) for the detention of up to 24,000 people would be built in “remote parts of the country”.

Whether the pivot was also the result of focus grouped thinking from would-be voters is not clear – though certainly the party has the sort of war chest to fund such research.

However, what cannot be discounted is the battle for a not insignificant number of voters considering a vote either for the Greens or Reform – parties which on paper are diametrically opposed but both present as populist change-agents.

Reform’s policy has not gone unnoticed among Green activists pounding the streets in areas where the party believes it is in a strong position to benefit from voter desire for a change.

“It hasn’t come up when we knock on doors here and talk to people who are – quite obviously – much more concerned about bread and butter things,” said James Meadway, a one-time adviser to former Labour shadow chancellor John McDonnell, who is now standing to be a Green councillor in the Bromley North ward of Tower Hamlets council.

At its root, Meadway saw Reform’s policy as an attempt to speak to its core voters. But he added: “The other thing we are seeing is that even where we are finding people who are torn between voting Reform or Green, or not voting. We’re talking about people who are upset at the state of the world and who want something to change.”