Tory strategists are worried the arrival of Donald Trump for a Nato summit in London on Tuesday is a moment of potential danger, fearing what the unpredictable US president will say or do.
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On Friday, Mr Johnson took the highly unusual step of urging him not to intervene in the election campaign – knowing his backing would backfire – and the pair are unlikely to hold a face-to-face meeting.
The BMG research shows the Liberal Democrats continuing their slide, down five points, while the Brexit Party is marooned on just four per cent, behind the Greens on five per cent.
Jo Swinson’s party had hoped to scoop up most Remain voters from the 2016 referendum, but the poll shows more drifting to Labour – 46 per cent, up from 39 per cent last week.
The Conservatives are worried the Lib Dems’ troubles are leading to the Remain vote coalescing behind Labour and its promise of a Final Say referendum.
Vince Cable, the former Lib Dem leader, criticised his party’s policy of revoking Article 50 – now all but ignored in its campaign – as always “wildly improbable”.
“We were never going to get 350 MPs, so the policy had been and should be to argue for the people’s vote, going back to the public, to the referendum with support for Remain,” he said.
Mr Struthers pointed to Labour voters coming home, with 73 per cent of those who backed the party at the 2017 election now planning to do the same on 12 December – up from 67 per cent a week ago.
In contrast, the Tories had “less room for growth”, having already banked most Brexit Party supporters, after Nigel Farage’s decision to pull out of Conservative-held seats.
Mr Johnson’s party is backed by 70 per cent of Leave voters, and although Labour has attracted only 17 per cent, that figure is up four points in a week.
“The key question for the remainder of the campaign is the extent to which Labour can continue to squeeze the Liberal Democrat and Green vote,” Mr Struthers said.
“If this trend continues, this election could be much closer than it looked just a matter of weeks ago.”
BMG questioned 1,663 voters between 27 and 29 November