The Liberal Democrats are drawing up an aggressive new election strategy targeting more Conservative seats, including that of foreign secretary Dominic Raab amid alarm among senior Tories about the threat posed by Jo Swinson’s party.

Research by the Lib Dems conducted over the summer has convinced officials to rip up the party’s existing plans and adopt a more ambitious targeting strategy after concluding that it was on course to win more than 70 seats – a result that would represent its best ever return.

Tory former cabinet ministers are among those now warning that the arrival of Boris Johnson and his acceptance of a possible no-deal Brexit has opened up a huge opportunity. “The route the PM and [his senior adviser] Dominic Cummings have taken is really blind to the fact that you’ve opened up this yawning chasm in the centre of politics,” said one. “The Lib Dems have always been at their best in a crisis.”

The new research has also prompted a scramble to raise the resources necessary to fight a more combative campaign. Swinson is understood to have begun a drive to win over new donors, including former Tory and Labour backers.

Surrey’s Esher and Walton, held by Raab, is said to be among the most ambitious seats on the radar, even though he has a huge majority of more than 23,000. The party is also searching for high-profile candidates to fight prominent Tories. The actor Emma Kennedy is said to be in the running to take on cabinet office minister Michael Gove in Surrey Heath.

The route the PM has taken is blind to the fact it’s opened up a yawning chasm in the centre of politics

An internal memo from June, written by the party’s campaigns director Shaun Roberts, said 76 seats were considered winnable, and a further five percentage point swing brought more than 200 seats into play. However, the party has slipped among some pollsters since the memo was written.

“Of the first 100 seats we can target, most of these seats will come from the Conservatives,” it stated. It called for an immediate “expansion of our target seat list” and emergency selection procedures to ensure candidates were in place by the autumn. It told party staff to “revise [the] general election campaign plan” and ordered new research to be conducted earlier than planned at the start of September.

Insiders said there was also an attempt to ensure a more diverse roster of candidates in winnable seats, while attempts to build momentum with further defections continue.

Several senior Tories said they now had serious concerns about the Lib Dem threat. One said it had been raised at a meeting of Theresa May’s cabinet, but largely dismissed as lacking credibility. Some in vulnerable seats believe they are being “written off” as Downing Street aims at the larger prize of securing a majority by uniting the pro-Brexit vote.

“There’s a really obvious home [for moderate Tories] … Look at Winchester – it is hugely vulnerable,” said the former cabinet minister. “It’s the sort of city where people will say, you know what, we don’t want to leave the EU, we don’t like this hard-right posturing of the likes of Dominic Raab and Priti Patel. Her approach to law and order goes down really badly. These voters want to rehabilitate people and get them out of prison.”

Dominic Raab, on his first trip to Washington as foreign secretary last week; his Esher and Walton seat is in the Lib Dems’ sights. Photograph: Zach Gibson/Getty Images

Another added: “There are lots of seats in the south-east where Conservatives are at risk.”

New research also suggests Johnson faces losing key marginal seats to the Lib Dems. A YouGov poll of more than 1,200 voters in 20 seats to be fought between the Lib Dems and the Tories found a 14.1 percentage-point slump in Conservative support and an overall swing of more than eight points to the Lib Dems compared with the 2017 election.

The polling, commissioned by the People’s Vote campaign for a second referendum, covered constituencies with the smallest Conservative majorities where the Lib Dems came second in 2017. Cheltenham, Richmond Park, Wells and Hazel Grove in Manchester would be among the seats to fall.

Peter Kellner, past president of YouGov and a People’s Vote supporter, said: “This polling shows that, in the battleground seats, Boris Johnson’s hard line on Brexit is far from a deal sealer in any early election. Even before a campaign has begun the Lib Dems could expect to benefit from enhanced coverage, and their tactical voting message will almost certainly gain traction. Jo Swinson’s party is well placed to make significant gains at the Tories’ expense.”