It’s a scene many commuters will be familiar with. The first noises of a commotion trickle down a carriage: a raised voice, perhaps an obscenity to turn the atmosphere blue. Passengers exchange knowing looks – this is a rare event when the strict anonymity of public transport is broken and strangers form a temporary bond.

“You think you can disrespect me because you’re Indian?” came the yell on a morning commuter train from Manchester to London this week. “This is my country!” I stand up to see a middle-aged, suited man walk straight up to an ageing passenger of Indian origin, finger pointed directly in his face. “Disrespect me and I will end you!”

Slightly miffed that no one else had intervened at this point, I told him to leave the carriage and take his racism with him. A reassuring murmur of approval from fellow passengers followed, and although Racist Suited Man barked something back, he retreated to the toilet. A couple of minutes later he re-emerged to yell: “I’m not a racist by the way!” provoking laughter of ridicule.

The episode disturbed me. Yes, of course racism – subtle or otherwise – did not vanish in the nineties and noughties. This was the sort of incident that could have happened at any point over the last few decades. And yet my fear is that forms of racism that had been driven to the fringes are now being legitimised once more and injected back into the mainstream.

Forms of bigotry that were once seen as publicly acceptable had become stigmatised. Because of the determined campaigning of anti-racism activists, many of us learned to treat each other with respect. Each generation, the evidence suggests, was more accepting than their parents. Yes, there has always been backlash – the “political correctness gone mad” brigade – but the direction of travel seemed pretty obvious.

No more, though. I can already hear the howls of protest from Ukip supporters. Here is another desperate attempt to shut down any discussion of immigration by the metropolitan liberal elite, they will cry. Ludicrous, of course, given that immigration has been one of the central issues of political debate for years now. Let’s keep on having that debate, but make sure all voices are heard. As other commentators have noted, it is actually those who resist the political elite’s desperate scramble to outdo each other on anti-immigration rhetoric who are shouted down. But look at where we have now got to as a country.

Mark Reckless, who is likely to be elected as Ukip’s second MP today, this week appeared to call for European immigrants to be repatriated. This is straightforward BNP rhetoric. Even a couple of years ago, for a mainstream politician to make such a public declaration would surely have provoked a national scandal. But the coverage was not of an intensity that matched the severity of the comments. The more these sorts of public declarations are uttered and faced with a weak response, the more the goalposts of acceptable political debate will shift, and ever more xenophobic and racist rhetoric will be normalised.

A few months ago, the British Social Attitudes survey suggested public attitudes on race had gone backwards. Racist incidents in the NHS were reported to have dramatically jumped at the end of last year. These are warnings that we need to heed.

It’s not just Ukip: it’s the broader Ukipisation of British politics. Last month, Tory minister Michael Fallon said British communities were being “swamped” by immigrants and their residents felt “under siege”. He was backed up by David Blunkett, the authoritarian former Labour home secretary. Last year, the government sent infamous “go home” vans to mixed communities, provoking condemnation even from Ukip. All major parties are now in competition over cracking down on immigrants.

Yes, let’s debate immigration. Let’s allow people’s fears and worries to be heard, and let both supporters and opponents of immigration be heard too. But Britain is heading towards a potentially disturbing moment. Language once seen as unacceptable provokes ever-smaller responses, and too few of us flinch. If it carries on, it will embolden many more, like Racist Suited Man, to berate people of other ethnic backgrounds for disrespecting them in “their” country. Who knows how ugly it could get.