I lost count of how many times Andrew Marr asked Ed Balls the question. Perhaps it was six, or seven, or even eight. Over and over it came: Will you rule out a post-election pact with the SNP?

Marr's line of questioning showed how comprehensively the media has swallowed the Tory election line. Quite probably Lynton Crosby has overestimated how troubling the notion of Salmond wielding influence in Westminster is to English voters, but regardless – the strategy is go. Every time Labour front benchers are questioned – from the Tories in parliament or the press outside it – they are asked to rule out a pact with the SNP.

Ed Balls failing spectacularly to conclusively rule out a post-election deal with the SNP. #marr — Peter Dominiczak (@peterdominiczak) March 15, 2015

That interview demonstrated Labour really do need a better line on this SNP Q. However unfair Labour think it is, it isn’t going to go away — James Forsyth (@JGForsyth) March 15, 2015

Balls, who not long ago ruled out a coalition with the SNP, refuses to do so for confidence-and-supply. There's a very obvious reason for that: it would effectively finish Labour's chances of winning the election.

No party is going to secure a majority after the election. It's very likely neither Labour nor the Tories will be able to govern even in a two-party coalition with the Lib Dems. David Cameron can reach out to Unionist parties and Ukip. Ed Miliband has the option of confidence and supply with the SNP. Asking him to rule that out is tantamount to asking him to give up the election.

The SNP are on course to achieve a historic victory north of the border. For Labour to rule it out would also rule them out of Downing Street. Morally it would also be hugely problematic. With such a tide of support flowing to the nationalists from all over Scotland, ruling out a pact would quasi-disenfranchise the country. It's also profoundly damaging to the Union, because it confirms Scots' suspicions that their political choices are not respected in Westminster.

To his credit Marr later asked George Osborne whether he would rule out a deal with Ukip. The chancellor answered with much of the same bluster as the shadow chancellor had a few moments earlier. With both parties slumped in the low thirties and neither establishing anything approximating a convincing lead, they are not likely to want to start ruling out avenues to power on May 8th.

And yet this question of a Tory-Ukip deal has seen precious little attention from elsewhere in the press. While journalists swallow the Tory elections lines on Labour and SNP it is hard to discern any such questions being asked of the governing party and its eurosceptic challenger.

The stock response to this observation is to say that the SNP wants to dismantle the country, whereas Ukip does not. It's plainly true that the SNP supports independence, but parliaments survive pretty easily while hosting those who wish them ill. After all, Nigel Farage is able to operate in the European parliament without the sky falling down. If Ian Paisley and Martin McGuinness can work together, Westminster will survive an SNP deal with a governing Labour party.

Ukip, on the other hand, promise an attack on Britain's political and constitutional arrangements which is disproportionate to the size of their relatively modest support. It's clear Ukip would pull out the EU, but it is not clear what it would do with the 2.34 million EU citizens living in the UK. Mark Reckless suggested they would be told to the leave the country, although Farage seemed to rule that out. Either way, pulling out of freedom of movement arrangements would eventually mean that the 1.8 million Brits living in Europe, most of them past working age, would be sent home. It would be an economic and demographic disaster the likes of which Britain has rarely seen.

To even those sympathetic to the party, it must be admitted that Ukip is beset by racism, sexism and homophobia. Farage himself said he would feel uncomfortable if Romanians moved in next door, called for the end of laws against racial discrimination and branded Muslims a "fifth column". The party's candidates, councillors, MEPs and peers have variously said that Lenny Henry should "emigrate to a black country"; that Muslims have a "totalitarian ideology", should sign a special code of conduct and "are breeding ten times faster than us"; that "forced repatriation" of immigrants should be considered; that immigrants should be checked for "communicable diseases" on entry; and that "no employer with a brain in the right place would employ a young, single, free woman". Ukip councillor Rozanne Duncan was filmed saying she really did "have a problem with people with negroid features".

I've sat in kitchens with European and non-European immigrants, all of them professional people working in this country for years, and listened to them make plans to leave the UK if there is a Tory-Ukip pact. While the press is prepared to treat Ukip like a lovable old uncle who drinks a bit too much sherry at Christmas, the rest of the world sees it for what it is: a hard-right nationalist party with some pretty poisonous policies and very questionable candidates. Any deal which would see Ukip in Downing Street, in any capacity, would deliver a devastating blow to Britain's international reputation and significantly destabilise its social make-up and economy.

None of which is to say that Cameron should rule out a coalition with Ukip. No-one should be ruling out a coalition or a confidence-and-supply agreement with anyone, unless it is in their electoral interests to do so. It is up to the public to decide who to vote for. It's not for politicians to wall off voters' options according to their own sensitivities.

But if Labour are going to be asked incessantly whether they will rule out an SNP pact, then the press might as well have the common fairness of asking the same question about the Tories and Ukip. Their plans for the country would certainly do far more damage than those of the SNP.