Even before he was formally sworn in as Scotland’s first minister, Humza Yousaf had made his first mistake. Not offering his defeated rival Kate Forbes a senior post in his cabinet and is a big error that will return to haunt him. The former finance secretary rightly turned down a demotion to the rural affairs brief. The offer that was made to be refused made a nonsense of Yousaf’s pledge to unite the Scottish National Party, after bitter divisions were cruelly exposed by its bruising five-week leadership contest. The SNP has been the most united and disciplined party I have seen in my 41 years on the Westminster beat.

Until this contest, it was virtually impossible to get its politicians to criticise each other — even in the bars or over the lunch table. In comparison, Tory and Labour figures are easy meat. Nicola Sturgeon even survived the destabilising feud with her mentor, Alex Salmond. The glue that held the SNP together until Sturgeon’s resignation was the holy grail of independence. It eclipsed the differences over economic and social policy that exist in all parties. Now the genie is out of the bottle: the party’s first contested leadership election since 2004 suggested it didn’t agree on much else other than independence. Yousaf will struggle to unite his party, not least because he does not enjoy a strong mandate. Forbes won 48 per cent of the vote and will now be a dangerous presence on the backbenches. The “Tartan Tories” who dominated the SNP before Salmond diverted it down a social democratic road, are suddenly more powerful than they imagined.

Although Yousaf was the Sturgeon “continuity candidate”, his grip looks fragile. Forbes received two-thirds of the second preference votes of members who backed Ash Regan, the third candidate. Of those Yousaf supporters who gave a second preference, 63 per cent opted for Forbes. That could provide a base for a future leadership bid if Yousaf lives down to expectations. Some SNP figures already fear he will prove to be “our Liz Truss.”

Yousaf rightly hinted at a more consensual, inclusive leadership than the top-down style and tight inner circle under Sturgeon. But he risks ignoring the voting figures and has appointed as his deputy Shona Robison, dubbed “the first friend” of the former first minister Sturgeon. (Robison insisted today that Forbes wanted a “better work-life balance” but that doesn’t fit with Forbes’ comments during the campaign).

Sturgeon, a brilliant communicator, will be a very hard act to follow, but her successor’s daunting in-tray reminds us she had run out of road. If one of the most gifted politicians of her generation could not secure a second independence referendum, how can Yousaf? He does not have a Plan B, after Rishi Sunak inevitably rejected his call for one in their telephone call last night. Yousaf has hinted at a gradualist campaign to build support for independence and has wisely ditched Sturgeon’s plan to turn the next general election into a “de facto referendum” (which, whatever the result, would not secure one).

But he will feel under pressure from SNP activists to keep the dream alive and may fall into the trap of raising false hopes, which eventually caught up with Sturgeon after eight years of somehow keeping the show on the road. Indeed, his first comment on leaving the stage after his election victory on Monday was to demand the power to hold a referendum. Sunak’s polite but firm refusal right reflects the “kill the Nats with kindness” strategy of Michael Gove, rather than the reckless, confrontational approach of Boris Johnson and Truss, which played into the SNP’s grievance politics.

It is premature to write off the SNP. All parties are coalitions and perhaps it can rediscover the glue that held it together for so long. But Yousaf’s uncertain start suggests his dream job – he called himself “the luckiest man in the world” – might turn into a nightmare. Support for independence has dipped to barely above the 45 per cent level in the 2014 referendum and may have passed its high watermark. Voters regard health and the cost of living as the priorities and have finally tired of the SNP’s patchy performance in 16 years of power. Labour, the Tories and Liberal Democrats are already weaponising Forbes’ attacks on Yousaf’s own record during the contest, when she warned against “acceptance of mediocrity.” They already have their attack lines for next year’s general election; Yousaf’s first test at the ballot box will be a hard one. The SNP will struggle to maintain its current tally of 45 seats. Labour will be the biggest beneficiary; insiders tell me they now have their eyes on between 12 and 20 SNP-held seats, which could reduce the swing Labour needs for a majority from 14 per cent to 9 per cent. Labour’s best pitch: not to go head-to-head with the SNP on the constitution but to argue it can oust the Tories and improve the public services the SNP has failed.