Cathcart is a neighbour to Nicola Sturgeon’s Glasgow Southside constituency. To have both the leader and depute based in the same small patch wouldn’t look clever. It would make the SNP, hoping to regain ground lost to the Tories in the rural north and east, appear avowedly urban and west coast.

Mr Robertson wisely noted in his resignation that his role was aiding MsSturgeon “as Westminster SNP leader and as a parliamentarian representing a rural constituency”.

The leader and depute need to have demonstrable differences, as well as productive harmonies.

There is also frustration among SNP MPs with “up the road” – party HQ and the Holyrood operation – appearing to treat the Westminster group as an afterthought. An MSP as depute would increase the strain.

So my money is on an MP getting the gig. Among those taking soundings are Pete Wishart, Dr Philippa Whitford, Tommy Sheppard, Joanna Cherry QC and Kirsty Blackman. Westminster leader Ian Blackford is keeping his powder dry for now, but is also widely expected to stand when nominations open next month.

Indeed, it’s hard for him to avoid it. Mr Robertson set a precedent by showing a person can be both depute and Westminster leader. If Mr Blackford didn’t stand, it might look like a failure of nerves, or a skeleton rattling in a cupboard.

Based on the last two contests, the most high-profile MP ough to win. Treasury spokesman Stewart Hosie cantered home in 2014 against MSPs Keith Brown and Angela Constance. And two years later, Mr Robertson, then Westminster leader, romped it in the first round of voting against Mr Sheppard, MEP Alyn Smith and Inverclyde councilor Chris McEleny. So logic points to victory for Mr Blackford.

However things are a bit more complicated. I like Mr Blackford. He’s smart, affable, a hard worker. In 2000, as SNP treasurer, he had the gumption to stand up to Alex Salmond about the state of party finances. But he also has baggage. Lots of it. The sort that will repel a good chunk of the SNP members who will choose the winner.

There are echoes of last year’s Scottish Labour leadership contest. Anas Sarwar appeared the obvious frontrunner. He was better known and more polished than his left-wing opponent, Richard Leonard.

But in the end Mr Sarwar was comprehensively thrashed, dragged down by charges of hypocrisy and poor political choices that came back to bite him. The kids at private school, the shares in a tax haven, the £5m stake in a family firm without formal union recognition and which didn’t pay Labour’s real living wage, not to mention the hundreds of thousands pocketed from unearned dividends.

Mr Leonard didn’t have any of that, and it became his signal virtue. He was different, he was authentic, he wasn’t an entitled rich kid from a New Labour dynasty.

Mr Blackford’s baggage also concerns money and poor choices. A former banker who advocates tax cuts to stimulate growth, he is more in tune with the Salmond school of economics than Ms Sturgeon’s “progressive” tax regime. He likes his dosh too, pocketing almost £50,000 from outside earnings on top of his £76,000 basic as an MP.

He is paid £36,000 for 32 hours per year as chairman of funeral plan firm Golden Charter Trust Ltd, plus £1500 per day “for any additional work”. He also declares £12,000 a year as chairman of telecomms business Commsworld Plc.

In 2015, Mr Blackford also took £3000 from David Craigen towards his general election campaign in Ross, Skye and Lochaber. Mr Craigen is a millionaire hedge fund manager with Lansdowne Partners. Described by Mr Blackford as a “friend”, Mr Craigen is also a Tory, no small sin in many SNP eyes. He gave the Tories £50,000 in 2007 – the same year Lansdowne made £100m betting on the collapse of Northern Rock and other British banks – and another £9000 in 2008. For good measure, the parent company of Lansdowne Partners is registered in the Cayman Islands. When Mr Blackford was asked about Mr Craigen’s donation, he said he had “no knowledge of him being a Tory. He just gave us a small donation.”

But two years later he did it again. He accepted another £3000 from Mr Craigen towards his 2017 general election campaign, in full knowledge of his background.

Mr Blackford also employs his step-son as a senior caseworker. The pay is up to £37,184 in public cash.

All of which puts Mr Blackford in a bit of a spot. If he decides it’s too risky to stand, it will be read as weakness, and he may be challenged for the Westminster leadership. Rivals are waiting for a moment.

But if he stands and goes down in flames like Mr Sarwar, it could look like a vote of no confidence by the party membership, and his rivals will pounce all the same.

It’s the sort of sweaty dilemma that makes for great political drama. Whether Mr Blackford enjoys it as much as the rest of us, or whether it helps the SNP, is another matter.