The Lib Dems should step back in Boris Johnson's constituency to give Labour a chance of booting out the Prime Minister, more than 100 pro-EU groups say.
They have written to Jo Swinson urging her to back Labour in Uxbridge and South Ruislip by stepping back.
In return, the 134 groups hope Labour will give the Lib Dems a clear run at Foreign Secretary Dominic Raab in Esher and Walton - a pro-Remain seat.
The 'Boot Out Boris' letter has been signed by campaigners who 'want to change the dynamics of the election'.
Ms Swinson has also been encouraged to pull out of the race for Iain Duncan Smith's seat, where Labour are challenging his 2,438 majority.
It comes amid growing calls to vote tactically in order to prevent the Tories winning a majority.
The letter to Ms Swinson says: “We want the Lib Dems to back Labour in Uxbridge and South Ruislip, where there’s a real chance to kick Boris Johnson out.
"The 7 per cent you are polling there could be the difference between success and failure.”
It continues: "We know we are asking you a lot to help Labour without any reason to think Jeremy Corbyn will return the favour.
"But if Johnson wins a majority, he'll drag us out of the EU. So we implore you to look to the national interest.
"We also know you are already only running a paper candidate in Uxbridge. But we want you to go further and support Labour there.
"A similar letter is being sent to Corbyn asking Labour to support the Lib Dems in Esher and Walton, where you can bring down Dominic Raab.
"But to be clear, we don't want you to wait for him to say yes. Time is terribly short, so we want you to take unilateral action."
It adds: "To boot out the Tories, we have to vote tactically for the candidates with the best chance of beating them. If you step back where you don't stand a chance, that message will be heard loud and clear across the land."
Signatories include Devon for Europe, Sixteen Million Rising, British in France, Yes to Europe, Wales for Europe and Women for Europe.
Hugo Dixon, deputy chairman of the People's Vote campaign, said: “We are not just trying to boot Boris out in Uxbridge. We want to change the dynamics of the election.
"If the Lib Dems and Labour pull back in key seats, this will supercharge the message that we need industrial scale tactical voting.
"With that, we can deny Boris his majority, get a referendum and stay in the EU."
Alex Pilkington, chairman at Devon for Europe, said: "If the Lib Dems and Labour pull back in key seats, this will supercharge the message that we need industrial scale tactical voting.
"With that, we can deny Boris his majority, get a referendum and stay in the EU."
