For last two days, as the Leave campaign has taken the lead, the Remain side has shifted emphasis to the "positive case" for EU membership. David Cameron and George Osborne retreated to allow Gordon Brown and Labour to issue a five-point plan (would it be any other number?) for reform. But today "Project Fear" is back with a vengeance. For the first time in the campaign, Osborne and his predecessor Alistair Darling will share a platform to warn of an "emergency Brexit Budget" if the UK votes Leave.

Based on the IFS's forecast of a £20-40bn black hole, the sepulchral pair will set out £30bn of "illustrative" austerity measures. On the tax side, they will warn of a 2p rise in the basic rate, a 3p rise in the higher rate, a 5p rise in inheritance tax and a 5 per cent rise in alcohol and petrol duties. On the spending side, they will warn a £2.5bn cut to the NHS, a £1.2bn cut to defence, a £1.15bn cut to education and a £2bn cut to pensions.

"As Chancellor, I would have a responsibility to try to restore stability to the public finances and that would mean an emergency Budget where we would have to increase taxes and cut spending," Osborne will say (though he won't be delivering any Budgets if Brexit happens). “I am even more worried now than I was in 2008," Darling will declare. "As a former Chancellor, I have to tell you that would mean an emergency Budget where we would have to increase taxes and cut spending. Why on earth would we inflict that on ourselves all over again?"

It isn't pretty - but "Project Fear" is still Remain's best card. In recent days, the myth has resurfaced that "The Vow", the promise of further powers for the Scottish parliament, won the 2014 independence referendum. It didn't. An Edinburgh University study found that only 3 per cent of voters cited the offer as the main motivation for their decision. By contrast, 26 per cent named the fear that "independence would make Scotland worse off", 28 per cent the Yes campaign's "unanswered questions", 5 per cent the "risk" of independence and 5 per cent their lack of trust in Alex Salmond. It wasn't "The Vow" that won the referendum for the Unionist side, it was the much-maligned "Project Fear".

It is this strategy that Remain has long sought to emulate. Eleventh-hour promises of "reform" won't defeat Leave (as Lynton Crosby likes to remark, "you can't fatten a pig on market day") - but fear might. The calls by Tom Watson, Yvette Cooper and Ed Balls to limit free movement won't halt the drift to Brexit. David Cameron tried - and failed - to achieve change and Jeremy Corbyn doesn't want to. "We clear the decks for Labour to make a Labour case and they start falling out over immigration. Fucking moronic," a Remain source fumed. By raising the salience of immigration, Labour politicans risk merely encouraging voters to back Leave - the only side that can credibly promise an end to free movement (at the cost of leaving the Single Market).

Far better to hammer the message that reducing immigration isn't worth "wrecking the economy" for. Below the headline figures, as YouGov data shows, voters think that leaving is riskier than remaining and that the economy will be worse off post-Brexit. It is these sentiments that Remain needs to obsessively animate in the final week. Only when voters truly fear the consequences of their actions do they pull back from the brink. In the view of some, "Project Fear" hasn't been fearful enough.

But Osborne and Darling's "Brexit Budget" is the start of a closing rally that will leave voters in no doubt that they will pay - quite literally - for Brexit. If "Project Fear" can't win the referendum, then nothing can.