The first week of the UK’s election campaign ended with the unusual sight of senior Conservatives claiming they do not believe the opinion polls and insisting that Theresa May could be heading for defeat.

With some polls giving the Tories a lead of more than 20 points over Labour the pessimism might seem extraordinary: in fact, the gloom is highly tactical. To win a thumping victory, Mrs May must first scare voters into thinking she could lose.

“We take no great comfort from our position in the published polls,” said a Tory campaign official. “There is clear historical evidence that the reported Tory lead could actually be non-existent.

“If the vote is distributed unfavourably, we could easily lose our working majority. We need only lose six seats and no party would hold a majority.”

The calculation by Sir Lynton Crosby, her campaign chief, is simple: voters are more likely to cling to Mrs May’s offer of “leadership and stability” ahead of Brexit if they think Jeremy Corbyn has a real chance of walking into Downing Street on June 9.

Mrs May’s warnings about a “coalition of chaos” has been a signature message of her campaign so far, a suggestion that a vote for any other party, such as the Liberal Democrats or the SNP, risks putting Labour and Mr Corbyn into power through the back door.

Tory officials say that Sir Lynton and his campaign colleagues discussed on Thursday a blog by Nate Silver, the US elections statistician, who said that UK polls were “terrible” and that the UK snap election was “riskier than it seems”.

Sir Lynton’s tactics are not new. At the 2015 election he presented David Cameron as a bulwark of stability against the risk of another left-leaning coalition, depicting Ed Miliband as weak and at the mercy of the SNP’s Nicola Sturgeon.

Mr Corbyn is an even more tempting target for Sir Lynton. The Corbyn factor explains the bizarre sight of Mrs May trying to persuade voters she might lose, while her opponents try to paint the prime minister as invincible.

Tim Farron, the Lib Dem leader, announced on the first day of the campaign that the limit of his ambition was to provide “effective opposition” to Mrs May, conceding defeat to the Conservatives at the outset. Instead, the arc of his ambition is to reduce the size of the Tory majority and be a significant opposition party.

The last thing Mr Farron wants is for voters to entertain the idea he might join a coalition government led by Mr Corbyn. “Our focus groups find that people think Corbyn is a nice man but incompetent,” said one aide to Mr Farron. “We have to show we are a more competent opposition than Labour. We are inviting voters to choose their opposition — when we present that argument to focus groups, that message goes off the charts.”

Sean Kemp, a former adviser to Nick Clegg, says: “If Labour has no chance of winning, it does help people to switch to us. There is no guilt attached.”

Meanwhile some moderate Labour MPs are also adopting the desperate tactic of insisting that Mrs May is unbeatable, hoping to reassure voters they can vote for a competent local MP without any risk of Mr Corbyn becoming prime minister.

John Woodcock, MP for Barrow, wrote in the North West Evening Mail: “In the unlikely event Mr Corbyn was in a position to try to form a government (and let’s face it, we are 20 points behind in the polls under his leadership, it is not going to happen) I would refuse to endorse him as prime minister in any parliamentary vote that would follow.”

Another Labour MP said that many of his colleagues were telling voters on the doorstep not to worry about the risk of Mr Corbyn winning the election. Another said: “We simply try not to talk about him at all.”

And all the while, the Conservatives are drawing up a target list of more than 70 Labour seats where the sitting MP has a majority of less than 7,000.

It is a strategy intended to deliver a Tory landslide, but Mrs May is obliged to say she fears defeat. Meanwhile, Lib Dems and moderate Labour MPs believe their best hope is to tell voters that Mrs May is a dead cert to win.