It can only be a matter of days before we see the first Conservative poster showing Jeremy Corbyn in Nicola Sturgeon’s jacket pocket.

May, speaking in the Commons today, dusted down the Tories’ 2015 election playbook as she warned voters against electing a coalition of losers from Labour, the Liberal Democrats and the SNP. No surprise there. After all, she has recruited the same campaign supremo, Sir Lynton Crosby, an Australian expert who helped David Cameron in by warning that Ed Miliband would end up – literally – in the pocket of Alex Salmond, the then SNP leader.

Labour tried but failed to change the music. They never answered the Tory charge that Miliband would need the votes of SNP MPs to become prime minister – not least because it was true, as the best political attacks are.

May told the Commons that the Labour, Lib Dem and SNP leaders “want to unite together to divide our country and we will not let them do it.” The Tories believe that Corbyn will be a softer target than Miliband. Yet May’s carefully calculated risk of calling an election still carries risks for her party.

For some Tory MPs, the election does not feel quite so straightforward as it did immediately after her surprise announcement on Tuesday. Some who face a strong challenge from the resurgent Liberal Democrats are jittery about the growing calls for tactical voting against candidates who support a hard Brexit.

Theresa May attacks progressive alliance of Farron, Sturgeon and Corbyn

Cooperation between Labour, the Lib Dems and Greens at grassroots level was already taking shape in some areas before May announced the election. Compass, the democratic left pressure group, planned to launch a campaign for an anti-Tory progressive alliance after next month’s local elections. Neal Lawson, its chairman, said: “Only a progressive alliance, happening more from local deals not to stand or not to campaign, can stop the Tories and hard Brexit. As in the Richmond Park by-election, the people are ahead of the politicians on this.”

Gina Miller, whose legal action forced May to seek Parliament’s approval to start Brexit negotiations, plans a crowdfunded nationwide tour to campaign against “extreme Brexit” and the election of MPs committed to a “real” parliamentary vote on the exit deal.

Stephen Dorrell, the former Tory Cabinet minister who chairs the European Movement, said: “We urge voters to support candidates who pledge to ensure that we continue to develop our relationship with these [EU] neighbours and play a full part in the life of the continent in which we live. It is not too late to change our minds."

Tony Blair believes May has called the election now because she fears voters will come to regret their decision as the cold reality of Brexit dawns. He is urging people to support candidates prepared to keep an open mind about Brexit and vote against a bad deal. Blair may well be right that “regrexit” will now come too late, and that a re-elected May will claim a mandate for “Brexit at any cost”.

The former Prime Minister insisted he is not advocating tactical voting or an anti-Tory alliance. His qualification goes to the heart of the problem facing pro-Europeans: how to maximise support inside the straitjacket of our outdated first-past-the-post voting system. At the 2015 election, 11.3 million voters backed the Tories and another 3.8 million for Ukip. Although a total of 14.4 million voted for pro-European parties in Labour, the Lib Dems, SNP and Greens, Cameron still won an overall majority.

Although we will see some cross-party anti-Brexit campaigning on our TV screens, I suspect the impact when the votes are counted on 8 June will be minimal. The Greens are up for cooperating in a small number of seats, but an alliance would threaten the Tories only with the backing of the Labour and Lib Dem leaderships.

Angus Robertson says Theresa May is 'running scared' of Nicola Sturgeon

Such tactical voting helped Blair win his landslide 20 years ago next month. Crucially, it had the tacit support of Blair and Paddy Ashdown, the then Lib Dem leader, who were so keen on cooperation that they seriously discussed merging their two parties.

No such top-level support for an anti-Tory or anti-Brexit alliance exists today. The Lib Dems may have only nine MPs but the election gives them an unexpected springboard. They judge that talking about doing deals with Labour in a hung parliament would send some potential supporters back into the Tories’ arms.

Unlike Labour, the Lib Dems have a clear line on Brexit and will not throw in their lot with Corbyn, who they see as unelectable. They blame Remain’s defeat in last year’s referendum on his lukewarm support. The dislike is mutual. Although some Labour Party members’ anger at the Lib Dems for sleeping with Tory enemies in the 2010 to 2015 Coalition is fading, Corbyn is not one of them.