Both Theresa May and Jeremy Corbyn were posed it last week, and found the rarest of accords in refusing to answer, playing that old get-out-of-jail card of declining to address a hypothetical question.
Ask the same question of ordinary voters though, especially those who favoured leaving the EU, and you’ll find not the evasion of the professional politician schooled in skirting awkward topics, but uncertainty that they did the right thing.
This struck me powerfully a few days ago in conversation with a group of lifelong Labour voters in Cleckheaton.
We were overdue to get together, since the last time we’d met was the weekend after the EU referendum. Then, they were cock-a-hoop at the result, full of praise for the then-Ukip leader Nigel Farage because he had effectively forced the issue to the vote, and convinced that they had done their bit to free Britain from the shackles of an organisation changing our country for the worse. They were emblematic of the silent majority who voted for Brexit – hard-working people in urban constituencies concerned about uncontrolled immigration and worried for their jobs because of globalisation.
There was something else, too – a deep-seated frustration that politicians were ignoring their concerns. Here, at last, was a chance to let them know in no uncertain terms that the country was sick of it.
Even though unshakeably loyal to Labour, they ignored their party’s lukewarm endorsement of remaining in the EU, and voted for Brexit.
Hindsight is a wonderful thing, but to all of them the easy certainties of summer 2016 look rather less certain now. The likes of Boris Johnson, Michael Gove, and yes, Nigel Farage, stand exposed as slippery purveyors of untruths and are now regarded with a visceral dislike.
There isn’t Â£350m a week extra for the NHS, which moves with fatalistic inevitability towards yet another winter beds crisis, nor any end to immigration in sight.
The notion that Britain would bid a friendly, but firm, farewell to the EU and cruise into a new era as a global trading giant looks like a fantasy. The grimmer reality is deadlocked talks with the EU, a Government fighting like rats in a sack over dogma and unable to decide how Brexit should work, and a Prime Minister so weak that she will appear almost a figure of pity to the other European leaders she meets later this week.
These concerns resonate powerfully with my friends in Cleckheaton, as they do with voters just like them across Britain. This is not what they voted for, a chaotic process and a Government that gives every indication of not having a clue what the outcome will be.
They have seen not a shred of evidence that a better, more prosperous Britain will emerge at the end of it. Instead, they’re more worried than ever for their jobs.
The gnawing suspicion that Brexit is not going to turn out well adds to a sense of having been cheated, of not being told the full story before they went to the ballot box.
It would be going too far to characterise this as “buyer’s remorse”, a conviction that they voted the wrong way, but that’s the direction in which their feelings are moving.
What happens if that journey continues is as intriguing a question as how they would have voted in the referendum if they knew then what they know now, especially as supporters of a resurgent Labour Party that is listening to its grass-roots.
They believe their party would heed a call from its supporters for another referendum, and if that happened, young voters who have been energised and engaged with politics by Jeremy Corbyn’s leadership could provide a very different outcome.
That may bring howls of protest from ardent Brexiteers, and unleash a debate whose ferocity would make the current arguments between rival factions look like a playground squabble, but it is not beyond the bounds of possibility.
Labour has already said it will vote against a so-called hard Brexit which would see Britain leaving the EU without deals on trade and borders, an outcome which deadlocked talks point towards.
It is not a long stride from there to demanding that the electorate has a say on whether it supports such a stance, a call that would be likely to attract the support of the Liberal Democrats and the SNP, which would both fancy their chances against the weak and divided Conservatives, some of whose pro-remain MPs might rebel and back a new referendum.
Will this happen? That’s anybody’s guess, because how the whole Brexit process unfolds is impossible to predict. But if it does, an awful lot of people, and not just in Labour’s Yorkshire heartlands, are going to vote differently to how they did last year.