The sorites paradox is concerned with the question of when an object ceases to be an object. To demonstrate, a hypothetical mound of sand is depleted one grain at a time: at what point, after which grain, does the mound of sand cease to be a mound at all? This paradox is what now defines the status of those migrants to the UK who have traditionally voted Conservative, and the party loyalties of their children.

BBC debate: Corbyn hits out at Johnson's ‘racist remarks’ Read more

I often think of the depleting mound whenever the chancellor, Sajid Javid, appears, frantically trying to justify another racially suspect Conservative policy by pointing to his own brown existence within the party. I am sure that, at one point, Javid thought he was promoting a legitimate defence of the party he joined as an idealistic upwardly mobile Ayn Rand-loving student in the 1980s. In contrast, he has been tasked with defending today’s party against claims of Islamophobia, racism and draconian policies such as citizenship-stripping. At what point does Javid cease to be a loyalist, and turn into a pawn? At what point does the party that first captured his loyalty finally cease to exist?

The past 10 years in general, and the previous three in particular, have turned the heat up on members of, and voters for, the Conservative party who come from migrant backgrounds. After the Windrush scandal, the hostile environment and the coronation of Boris Johnson – a man whose entire career has been marked by racist insults and comments – the calculus of plausible deniability for the sake of free commerce and support of businesses no longer works. The party’s toxic policies can no longer be ringfenced or ignored by those who voted for it based on an affinity with conservative values in their countries of origin. An estimated 20% of ethnic minority voters voted Conservative in 2017 – a number that rose to 40% among British Indian voters – but it’s now getting hard to claim the Tory party will not detrimentally affect their lives.

In recent years, Conservative policy has made it extremely hard for non-EU citizens to come to, and live, in the UK. Voters from migrant backgrounds tend to have links to their or their parents’ countries of origin, rendering them direct victims of the hostile environment, increasingly cut off from relatives, and even spouses, abroad. It will be a challenge to find many such households where time and money has not been spent securing something as basic as a visitor visa for a relative.

A direct promise by Tory Brexiters during the EU referendum was that a leave vote would secure more working visas for south Asians to work in Britain’s curry industry. The votes were given, but the visas did not materialise. The message is loud and clear, the party will play on the commercial insecurities of such communities, then abandon them.

Religious and ethnic affiliations have also been manipulated. One of the lowest points in this country’s recent political history was Zac Goldsmith’s 2016 London mayoral campaign, which profiled voters based on their backgrounds and their propensity to vote Conservative, and sent British Indians leaflets claiming that Sadiq Khan was a danger to their community as he did not attend an event welcoming the Indian prime minister, Narendra Modi, and supported a wealth tax on family jewellery. David Cameron himself doubled down on the gutter tactics by sending a letter targeting Hindus and Sikhs in an attempt to convince them not to vote for Khan (who has Pakistani descent) because “closer ties between the UK and India have been a priority for me”.

Even if one were to ignore the race-baiting, for migrants who are more business-minded, Labour has now become the natural party of choice – by virtue of its soft Brexit position and anti-austerity economic strategy. Appealing to the entrepreneurial, free enterprise spirit of some migrants and their children no longer holds water at a time when the government is putting politics before trade. Brexit is predicted to harm small business owners more dramatically than large corporates. A third of the 1,000 companies surveyed by the Federation of Small Businesses in September said Brexit had already caused “either temporarily or permanently reduced profitability”.

“No one wants their politics to be assumed based on the colour of their skin,” wrote Conservative campaigner Binita Mehta-Parmar in 2015. “We second- or third-generation British Asians don’t possess the leftover feelings from the 70s and 80s, when many automatically associated with Labour as they championed anti-discrimination laws.” The bad news for these voters is that these “leftover feelings” aren’t mere political flotsam, remnants of a bygone time. They are accurate fears about a party that still views ethnic minorities as political pawns, pits them against each other and cynically stokes anti-immigration fears to capture votes. The subliminal party slogan is, “If you want another immigrant for a neighbour, vote Labour”, recalling the notorious Smethwick campaign in the 1964 election.

Grain by grain, what once distinguished the party has ebbed away: the fiscal responsibility and pro-business agenda has disappeared, leaving only the racism and opportunism. For Britain’s migrants and their children, a vote for Labour now is not a defection to another tribe, it is a matter of self-preservation.

• Nesrine Malik is a Guardian columnist