Even before Thursday’s jaw-dropping news that one of its MPs is joining the Conservatives, this year’s Scottish National party conference was bound to be the most challenging for years. As the first SNP conference since Humza Yousaf succeeded Nicola Sturgeon as leader, the first since the police investigation into the party’s finances, and the first since Labour’s sweeping victory over the nationalists in the Rutherglen and Hamilton West byelection, next week’s gathering in Aberdeen already had plenty of problems to navigate. The defection of Lisa Cameron to the Tories may seem at first sight to be much more manageable, given the SNP’s loathing of the Conservatives. Yet the charges that Ms Cameron levelled against the SNP as she quit cannot be brushed aside so easily. They carry too many echoes for that. The SNP is already facing a cold electoral wind for the first time in more than a decade. The loss of another MP could turn the headwind into something even icier. Ms Cameron jumped before she was pushed as the SNP MP for East Kilbride, Strathaven and Lesmahagow. Her disputes with the party leadership, and isolation within the SNP group at Westminster, were long signalled. The former NHS psychologist was facing imminent deselection this week, with much of the party hierarchy either openly or tacitly wanting her replaced. In his less-than-magnanimous response to her departure, Mr Yousaf called it the least surprising news he had heard as party leader. But Ms Cameron has in some respects done Mr Yousaf a favour. By crossing the floor rather than resigning, she helps avoid another byelection, in a neighbouring south Lanarkshire seat to Rutherglen, that the struggling SNP leader could not afford to lose. In the current state of Scottish politics it is hardly an opportunist career move for the MP to join the Tories, who languish a distant third in the general election polls north of the border. But it is surely a mark of her frustration with the SNP. It comes just at the time that Mr Yousaf’s leadership, and the party’s reputation, are already under more critical scrutiny than the SNP is accustomed to dealing with. Whether this will all explode in Aberdeen is unlikely. But the activists will notice, and so will the voters. In her resignation, Ms Cameron cited multiple personal and political differences. She called the SNP group culture “toxic and bullying” and bad for her mental health. When she supported the harassment victim of fellow SNP MP Patrick Grady, she said she was “ostracised” by party leaders who then rallied around Mr Grady. A practising Christian, she opposed both abortion liberalisation and the SNP’s gender recognition bill and supported the UK government’s intervention to challenge the latter in the courts. Above all, she hit out at the “division” caused by the SNP’s independence policy itself. Party leaders would be more able to dismiss these charges if they did not echo others. The SNP’s top-down centralism in the Sturgeon era has often been criticised for its intolerance of dissent. Other SNP MPs have also said they were marginalised if they disagreed with the party’s social agenda. Kate Forbes MSP, defeated by Mr Yousaf in this year’s leadership election, is the most prominent of these, but she is not the only one. Other MPs have lost the whip, joined Alex Salmond’s Alba or have already decided to call it a day when the next election comes. Whether or not it reveals itself fully in the carefully controlled environment of the Aberdeen conference, Scottish politics is going through a change moment. Independence remains popular, especially with young voters, but the SNP is being challenged more seriously than for many years. Mr Yousaf has his work cut out to steady the ship.