When sifting through my mail, unsolicited post from political candidates usually ends up in the same place as letters imploring me to make a claim for a non-existent injury or leaflets offering an extortionate loan.

But this one was different. The first detail that instantly struck me about this mailshot from the Tories on the London Mayoral election was the large photograph in the top-right-hand corner. It showed the party's candidate, Zac Goldsmith, shaking hands with the Indian Prime Minster Narendra Modi.

Intrigued, I decided to read on. The text below told me about Mr Goldsmith's gratitude for the immense contribution the British Indian community makes to London and what a deep love he has for India after travelling there in his youth. It then detailed why he would be the best mayor for all Indians, in sharp contrast to his Labour opponent Sadiq Khan (subtext: Khan is Pakistani).

I was, quite frankly, in a state of shock. Did Conservative campaign headquarters really think it worthwhile to trawl through the electoral roll and send such a blatantly sectarian letter to everyone in London with an Indian-sounding name?

As I continued opening my mail, I then came across a similar mailshot addressed to me, but this time bearing the photocopied signature of David Cameron. Again, it told me how Zac Goldsmith is a great friend of all Indians and will do his utmost to promote our interests.

Had it not occurred to the Tories that I may hold political interests and values based on my life in London rather than on where my parents happened to be born?

It was then that my emotions turned to anger. I felt deeply patronised and wholly insulted by the idea that I would vote for a party purely on the basis of my ethnic origin. Had it not occurred to the Tories that I may hold political interests and values based on my life in London rather than on where my parents happened to be born?

It turned out I was far from the only one to feel this way. It was not long before there were press reports accusing Zac Goldsmith and David Cameron of 'racially profiling' voters in their campaign material. Meanwhile scores of recipients took to social media to condemn such 'patronising crap' and a 'botched and lazy' attempt to engage with the Indian community.

Even after a period of sober reflection, it remains impossible to disagree with these sentiments. The most grating element is the thought that all British Indians should be treated as a single homogenous block. In grouping us into a broad ethnic category, the Tories failed to realise that – to whatever limited extent we may be governed by our cultural background – it is more likely to be a narrower religious or regional identity within the Indian umbrella.

So as a Punjabi Sikh, for example, nothing is more likely to make me vote Labour than a picture of Prime Minister Modi on a Tory pamphlet. He may be popular among many Indians, but he remains a figure of suspicion to us minorities given his Hindu nationalist past and questionable attitude towards Gujarat's Muslim population when he was chief minister of that state.

David Cameron's letter was at least a little more nuanced, as it outlined the different ways in which he appreciates the Gujarati and Punjabi communities. But again, this broad brush approach will always alienate some people on the basis of their ethnic identity.

The most grating element is the thought that all British Indians should be treated as a single homogenous block

As one recipient called Barbara Patel, who is actually of Jewish descent, reportedly wrote back to Downing Street: "You have made a number of assumptions based on my surname (Patel=Gujarat and Gujarat=Hindu) and have attempted to use these ethnically based assumptions to 'scare' me into voting for your candidate, Zac Goldsmith. I am not from Gujarat. I am not a Hindu, my husband's family are lapsed Muslims. Above all, I have never been, nor ever would be, a Tory voter."

Indeed, it is difficult to think of a greater campaigning own-goal than this particular endeavour.

It seems that this bungled, if possibly well-meaning, attempt to appeal to Indian sentiments stems from a question that has troubled the Tories for several decades now – why can't they win the Indian vote? They have long struggled to understand why a community whose predominant values of thrift and enterprise are so in tune with the Conservatives should continue to give most of its votes to the Labour party.

It is a fair question, and one whose answer lies in studying the historical conditions for immigrants in Britain. The first generation that arrived in the 1950s, 60s and 70s may have disagreed with many of Labour's punitive taxes and business regulations, but they had a clear feeling that the party was simply a lot less racist than the Tories. Not only that, but Labour also contained several figures who were genuinely tolerant of ethnic differences and keen to support them through a policy of multiculturalism.

This natural affinity with Labour was transmitted to subsequent generations, no doubt to the great frustration of Tory strategists who felt nothing had done more to promote British Indian wealth generation than Margaret Thatcher's unleashing of the free market.

But things have changed in recent years. The limited ethnic polling carried out on the 2015 general election revealed that the Tories are close to catching up with Labour in capturing the Indian vote, if not yet that of other Asian communities. This only came about once they made concerted efforts to shed the 'nasty party' tag and demonstrated they are intensely relaxed about the diversity of modern Britain. They even have Asian Cabinet ministers now.

In short, they have recognised us as equal human beings and citizens rather than outsiders defined by race or religion. That is all they need to do, as it marks the point at which cultural background ceases to matter to ethnic voters as a political issue. Beyond that, the parties are judged on their own terms, by their policies. If the Tories remember this basic fact, rather than regressing to crass racial categorisation, then they may attain their cherished aim of capturing the majority of the Indian vote by the time of the next general election.

Harcharan Chandhoke was born in Derby to Indian parents. He writes on issues affecting the Indian diaspora across sport, culture and current affairs.