Tory insiders fear a catastrophic defeat to the Lib Dems in the crunch North Shropshire by-election as Boris Johnson's authority takes a battering over No10 Christmas parties.
One senior party figure described the vote in the 'true blue' West Midlands seat as "a local referendum" on the Prime Minister - adding "and he's losing".
The ballot box test for the Conservatives follows weeks of Downing Street chaos, mounting public over rule-breaking festive dos and Tory sleaze allegations.
The vote was triggered after former minister Owen Paterson quit having repeatedly breached parliament's lobbying rules.
But despite the huge 22,994 Tory majority in 2019, bookmakers have made Lib Dem candidate Helen Morgan favourite to snatch victory.
The Conservative candidate, Neil Shastri-Hurst, is also battling strong campaigns from Labour's Ben Wood and Kirsty Walmsley, a former Tory councillor standing for new right-wing party Reform.
Losing the traditional stronghold, following the Lib Dems' shock win in the Chesham and Amersham by-election in June, would pile pressure on the PM after he suffered the largest Tory rebellion of his premiership after MPs lined up to reject his Covid 'Plan B' strategy.
One senior Tory told the Mirror: "I was there this week and it’s really grim on the doorstep.
"For the first time, it’s become a personality problem and it’s about Boris - the public have stopped laughing at the joke and just feel cheated.
"This is a local referendum on the PM and he’s losing."
Another Tory who was supportive of the PM admitted the vote was "on a knife edge" but the party may still hang on.
A Lib Dem source on the ground predicted a major upset in the "truest of blue Conservative safe seats".
"The fact that this is under threat is a damning failure of Boris Johnson," they added.
" Liberal Democrats have mounted a huge campaign here but we know this will come down to Labour voters lending us their support this time. We know it will be close but whether we come down on the right side of close will come down to polling day."
Other Lib Dem sources have said they are "nervous but optimistic" and that the result is "too close to call".
Meanwhile, the Labour camp, which is targeting the constituency's towns, said the Tories were "in for a kicking".
"We've had volunteers piling in during the week and we have dominated the local news agenda," said one campaign source. "We are confident we will do extremely well."
Keir Starmer's party has faced claims it has effectively deserted the field to give the Lib Dems a clear run, but sources underline Angela Rayner and numerous other shadow cabinet figures have been in the area campaigning.
Government minister Nadhim Zahawi played down the Conservatives' chances of winning in an interview with Times Radio on Sunday, saying by-elections are used to “send a message”.
He said: “By-elections have historically been used as a protest vote because people know it’s not going to change the outcome of which party governs the country but actually they want to send a message for whatever reason.”
The result is expected to be declared in the early hours of Friday.
