When Scots nearly voted to leave the United Kingdom in a September 2014 referendum, they sent a powerful message to those in favour of maintaining existing political and economic blocs in Europe: you can’t scare voters into line. Sadly, the campaign to keep Britain in the European Union still favours intimidation over persuasion, a tactic that may backfire badly when the polls open next Thursday.

Chancellor of the Exchequer George Osborne threatened yesterday that a vote to leave the EU would leave a “black hole” in British finances worth $42 billion (about £30 billion) — which he would fill with “an emergency budget where we would have to increase taxes and cut spending”. Unsurprisingly, the news has caused a storm of protest.

Some 57 Conservative members of Parliament have pledged to vote against any post-Brexit emergency budget, illustrating how divisive the European question is for the ruling party. More than 40,000 tweets on the topic have flown around.

The “Remain” campaign is running scared after five opinion polls in the space of less than 24 hours showed a lead for the campaign to leave the EU. Osborne may be correct that a decision to quit the bloc will cause an economic shock that will affect the budget and force the Government to make adjustments. But the way to get that message across is with data advertising the benefits of membership, not by menacing the electorate. The economic risks of Brexit have already been outlined by the International Monetary Fund, the Organisation for Economic Co-operation and Development, the Treasury, the Bank of England, and other bodies who have studied the likely short and medium-term economic impacts, and it has been repeated to voters constantly throughout the campaign. Voters are perfectly capable of drawing their own conclusions; Osborne’s gambit suggests he doesn’t think so. Yesterday’s speech makes him look like schoolyard bully. It verges on undemocratic: “Vote the way we want you to or we’ll take away more of your pay and we’ll cut the public services you use.”

It may well be the end of any hopes Osborne has of winning the Tory party leadership once David Cameron steps down as prime minister. By contrast, his chief rivals for the post, Michael Gove and Boris Johnson, increasingly sound like winners, focusing on the positive steps they would take after winning the referendum to disentangle Britain’s EU relationship.

For those of us who would prefer to remain part of the world’s largest single market, the widespread accusation that the pro-Europe campaign has become “Project Fear” is worrying — and sadly predictable. Here’s what I wrote in September 2014 after Scotland narrowly voted to remain part of Britain:

“Europhiles need to start accentuating the positives of EU membership now, and repeat the message insistently in the coming years. Cameron almost lost Scotland in large part because the unionist campaign focused almost exclusively on the negatives of separation; he must not make the mistake of trying to scare the electorate into sticking with Europe.”

That did not happen. Even though the economic evidence argues overwhelmingly for sticking with the existing EU relationship, the temptation to wield fear as a campaign weapon has proved irresistible. Cameron’s warning a few days ago that British state pensions could be at risk from Brexit was a particularly low blow. It plays on people’s fears about their personal economic futures, without any evidence that the present market uncertainty would be so exacerbated by a vote to quit the EU that their pension pots would be reduced.

The balance of power in the referendum next week rests with those who are undecided. An online poll of 2,497 adults by TNS published on Tuesday, for example, showed 47 per cent backing “leave” with 40 per cent for “remain.” That leaves 13 per cent who may still be persuaded to back the status quo. The way to do that, though, is to emphasise the benefits of staying — not by threatening to introduce a budget that makes people poorer.

• Mark Gilbert is a Bloomberg View columnist and a member of the Bloomberg View editorial board