This morning brings news that John Finnie MSP and Jean Urquhart MSP have left the SNP following the nail-biting and passionately-argued debate at their party conference over NATO. A decision probably thought to be without cost by the party’s Westminster group is proving extremely expensive indeed, and this is a major blow both for the party and, sadly, for the prospects for a Yes vote in the referendum.

First – it takes the SNP group down from 67 to 65 at Holyrood. Will the SNP regret pushing Tricia Marwick as Presiding Officer? A majority of nine became a majority of eight without her, then six when Bill Walker was expelled: now their majority is just two – 63 on the opposition side to 65 SNP, plus the PO’s casting vote. It won’t undermine the referendum bill itself, of course: both Finnie and Urquhart will still vote for it, as will Margo MacDonald, Patrick Harvie and Alison Johnstone.

Second – the pressure will now be on the other MSPs who supported the party’s previous anti-NATO policy. Some of the names are here on the defeated amendment: Sandra White, Marco Biagi, Jamie Hepburn, Bob Doris, Dave Thompson, and Gordon MacDonald. Remember other MSPs backed other options that would have blocked the change or at least postponed it: Christina McKelvie, John Wilson, John Mason and Rob Gibson, notably. I’ll add other names here if people have them. Goodness only knows what calls are being made right now to this list by the leadership. Two lost is a disaster – two more would be much worse, particularly for Angus Robertson, who should have known better than to try and divide his party on the strength of media pressure. The party backed him, narrowly, and he can’t be sacked: but this is his misjudgement, and he probably knows it. But the other pressures are enormous too. John and Jean have set a marker – that’s what a principle looks like, these resignations say: colleagues, do you believe in the principle too? Do you want to be remembered as the MSPs who argued for that principle but buckled when the leadership told you to? Reputations will be made and lost today.

Third – an informal grouping of five anti-NATO MSPs is already possible, including Margo and the Greens. Five seats gets you on the bureau. That challenges the the SNP’s one-party stranglehold on the business of Holyrood – one more departure would confirm that.

Fourth – it’s not coincidence that both John and Jean represent the Highlands and Islands. The strength of feeling in the Highlands and Islands branch has always been substantial: I’ve listened to Rob Gibson play anti-nuclear protest songs on his guitar, and if I were guessing who on that list will be feeling the heat, Rob and Dave Thompson are prominently placed.

EDIT Fifth – I was challenged on why this is bad news for the referendum, and realised I hadn’t explained that. The reason is, sadly, that as far as much of the media see it the SNP is Yes and Yes is the SNP: and the SNP themselves go out of the way to blur that boundary. Listen to the Who Dares Wins crowd telling everyone Scotland will now stay in NATO, rather than saying this is now the SNP’s new policy but that the decision will be taken the people after independence. The public don’t vote for divided parties, which the SNP now clearly are. I want a Yes vote, but hopefully it’s now utterly obvious to the SNP that they need to stop blurring the boundaries between themselves and the broader movement for independence: that movement remains united around the principle that Scots should make all the key decisions that affect them.

Since the bloodbath over selection and Swinney’s leadership in 2003, the SNP have run an extraordinarily tight ship. Four years of minority from 2007 onwards saw no rebellion of any significance whatsoever. Salmond has, for good or ill, commanded his party like a well-drilled regiment. This looks like it could be his greatest test.