Only two months ago, the talk among Tory MPs – echoing some newspaper front pages – was about how Boris Johnson wanted to “go on and on”, lasting longer than Margaret Thatcher’s 11 years in power. At that time, they dismissed the idea that he was a dilettante prime minister who really wanted to retire and write books, give after-dinner speeches on how hard the job was and make loads of money in the process. But that has all changed, with successive scandals over his handling of sleaze allegations and Tory insiders now openly questioning his future. Backbenchers are not just fuming about Christmas parties and Downing Street refurbishments, but also about the way, in their perception, Johnson has squandered the party’s poll lead and put their seats in danger. All eyes are on the byelection in the formerly safe seat of North Shropshire next week. It would take more than 50 MPs writing a letter of no confidence in Johnson – 15% of the 361 Tory seats in parliament – to trigger a leadership challenge. But an unexpected byelection loss could trigger more submissions to the 1922 Committee, the keeper of the letters. One MP summarises the situation thus: “Are there more than 50 MPs who have submitted a letter of no confidence in the prime minister? No. Are there more than 50 MPs who think he is doing the job extremely badly? Undoubtedly yes.” Most believe it is a long way off, with no knockout blow to Johnson yet so far, but any leadership challenge would be likely to come from a so-called stalking horse candidate, someone with no hope of winning who does not mind being vilified for being the first to wield the knife. After that, the serious contenders – the cabinet ministers Rishi Sunak, Liz Truss and Sajid Javid, or an outsider such as Tom Tugendhat – could potentially swing into action. Tellingly, all Johnson’s top team have been keen to distance themselves from the No 10 parties stories in recent days. On Friday morning, Truss, the foreign secretary, told the BBC: “Last Christmas, I was spending my time getting trade deals … We were busy working on trade deals.” Supporters of Johnson believe he can turn things around if he can make it through to Christmas with no further major revelations about lockdown-busting antics in Downing Street – but with rumours of other parties abounding, that may be a big if. One former Tory No 10 adviser was pessimistic: “He’s an agent of chaos, and he’s no fucking good at running anything. People are saying he needs to get a grip but that’s not going to happen.” On whether Conservative MPs would make a move against the prime minister, he said: “He got them their seats but he may cost them their seats. If we’re still in this situation in six months’ time I can see him going.” One former minister said they had dismissed stories about letters being submitted but had now begun to take the threat to Johnson seriously. “I’ve had about six emails from constituents. That’s not a flood, but MPs do start to listen when they come in from people who aren’t the usual troublemakers who hate you. Especially if they are personal.” Another MP said their colleagues would probably take some time to think during the Christmas break. “People will start to think: it’s only potentially a year till the next election. I need to think about my leaflets, my fundraising. And does Boris help or hinder? If people decide the latter, then we don’t hang about like Labour do.” Gavin Barwell, a former chief of staff to Theresa May who is now a Tory peer, has also highlighted a trio of issues that he believes have the potential to bring the prime minister down: the Downing Street parties, the flat refurbishment and the anger of backbenchers over a return to Covid restrictions. However, others believe the discontent among Johnson’s ranks is not as severe as it was for May, who was toppled by her own party. “The party is pretty ruthless,” one cabinet minister said. “But I am not getting the sense this is anything like the days before the no confidence vote in Theresa May. Cabinet has some disagreements but compare it to what came before. It’s chalk and cheese.” Even so, there was a consensus that Tory MPs would make a move if they felt the polls had turned convincingly in Labour’s favour or their own local areas appeared to be lost causes. A senior MP told the Guardian they believed the party had “already lost my seat” so felt “ungovernable”. Another said there had been an “erosion of authority”. “Boris has become politically toxic to the electorate,” the MP said. “They think the PM’s incompetent and a liar, and the parliamentary party isn’t far behind. He is incompetence personified.” Others in this camp said they were not going to put a letter in to call for Johnson’s resignation at this stage – but were gradually being convinced it might be necessary, if things deteriorated further. “The key is in the ignition,” said one of Johnson’s own ministers.