Senior staff at Conservative campaign headquarters are warning supporters against complacency as the party considers its response to favourable polling and a change in strategy by Labour.

Following predictions that Boris Johnson will reap a comfortable majority and Labour briefing that it will make more efforts to woo leave supporters, Tory sources said their main concern was that “inertia” could descend upon voters.

“If the polls are wrong by a few points, and they have been wrong before, we end up with a hung parliament. Our team know that, and now we have to convince the public. There should be no complacency or let-up or change of direction. We will continue with our central messages: get Brexit done, more hospitals, more nurses, more police,” a senior party source said.

A seat-by-seat YouGov poll on Wednesday predicted that the Conservatives would win 359 seats, Labour 211, the SNP 43 and the Liberal Democrats 13 if the election were held immediately.

That result would give Johnson a majority of 68, making gains at Labour’s expense in the Midlands and north of England. Labour would suffer its second-worst postwar defeat, with Jeremy Corbyn’s total two above Michael Foot’s in 1983.

The Guardian understands that Labour’s key strategists recognise that they now have much more to do to hold constituencies in Brexit-voting areas. They plan to present a pared-down set of manifesto commitments that will offer to put money directly in the pockets of voters in these constituencies.

Shadow ministers from leave constituencies are expected to be given more prominence at election events.

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The prime minister’s most senior adviser, Dominic Cummings, told the Conservatives to expect a drop in support before the election. In a lengthy blogpost, the former Vote Leave strategist wrote: “You will see many polls in the coming days. Some will say Boris will win. Trust me, as someone who has worked on lots of campaigns, things are MUCH tighter than they seem and there is a very real possibility of a hung parliament,” he said. “Without a majority, the nightmare continues. ALL other MPs will gang together to stop Brexit and give EU citizens the vote. It’s that simple.”

An analysis by the pressure group Best for Britain found that Johnson could be deprived of a Commons majority if just 117,000 people voted tactically to defeat the Tories.

Conservative strategists said they were planning more “get Brexit done” set-pieces next week, believing that the message has been central to their current position in the polls. This will be followed by announcements on defence policy in preparation for the 70th anniversary of Nato in London on 3 December.

“It will be an opportunity to show Corbyn’s weakness on Trident and the armed forces with traditional Labour voters,” said a Conservative party source.

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Party officials remained bullish about the possible appearance of more Labour leave shadow ministers during the campaign. “If the answer is Ian Lavery, you are asking the wrong question,” said one.

The party is braced for Donald Trump’s two-day visit for the Nato summit which begins on 2 December. The US president’s arrival will prompt further questions about Washington’s expectations regarding access to the NHS in any future trade deal.

A firm supporter of Brexit, Trump is expected to praise the UK’s decision on the trip, and to attract protests and controversy.

One Tory official said Johnson’s team were “braced” for his arrival during an election campaign. During his last visit, Trump alarmed his British hosts by criticising the way Theresa May was conducting the Brexit negotiations in an interview with the Sun.