IF Kate Forbes wins the SNP leadership election and the Scottish Greens refuse to work with her, will the Scottish Government fall?

Not necessarily, according to the UK’s top polling expert, John ­Curtice, professor of politics at the University of Strathclyde.

However there could be a ­difficulties if a large number of SNP MSPs also refuse to back her for the post of First Minister.

“If a lot of people sit on their hands, it is a problem,” Professor Curtice told the Sunday National. “For the ballot to be valid a quarter of the chamber has to vote so an enormous amount of people sitting on their hands would be difficult.”

Scottish Greens co-leader Patrick Harvie has already hinted his party will only work with Humza Yousaf and may pull out of the current ­power-sharing agreement with the SNP if Forbes, Yousaf’s chief rival, wins the SNP leadership election.

A number of SNP MSPs are also unhappy with Forbes’s views on gay marriage, abortion and the Gender Recognition Reform Bill.

The result of the SNP leadership election will be announced ­tomorrow and the Scottish Parliament will then move to elect a First Minister to ­succeed Nicola Sturgeon.

All the parties are likely to ­nominate their own leader, and one quarter of the Chamber has to vote in order for the vote to be valid.

“The crucial bit is given that there some SNP MSPs who are not too enamoured with Ms Forbes, will she manage to secure election as the First Minister? To do that, she has to get more votes than whoever ends up ­being the last candidate standing who is likely to be Douglas Ross,” said Curtice.

Ross’s Tory Party are the second ­biggest in the Scottish Parliament, with 31 MSPs. Labour have 22, the Scottish Greens have seven, the LibDems have four while the SNP have 64.

The first clue to how the ­election for First Minister is likely to pan out will be whether Patrick ­Harvie ­decides to stand, according to Curtice.

“If Yousaf gets it [the SNP leadership election] then Harvie won’t stand but if Forbes gets it, does ­Harvie stand as First Minister?

“They then have a series of ­successive ballots whereby the ­bottom person is thrown out and then that carries on until a candidate gets more votes than the votes cast for all of the other candidates standing. If it is Yousaf and the Greens back him we will only have one round of voting,” said Curtice.

“If it is Forbes and the Greens don’t go for her, then we may get more than one round.”

He said the first people to fall off the ballot were likely to be Lib Dem leader Alex Cole Hamilton, followed by Harvie, if he stands, “when the Greens would have to decide what to do next”.

If the Greens and Lib Dems ­decided to back Labour leader Anas Sarwar, he could knock Ross out.

If the MSPs don’t manage to elect a First Minister within 28 days of Sturgeon’s resignation last week, then there will be a Holyrood election.

However, Curtice, said: “Basically, assuming most SNP MSPs swallow their doubts and vote for Forbes, she is going to make it. How many Labour MSPs are going to vote for Douglas Ross?”

If Forbes is elected as First ­Minister and the Greens do pull out of the Bute House power-sharing agreement, she will be able to carry on as leader of a minority government – as the SNP previously did between 2007 and 2011 and from 2016 until the Bute House agreement with the Scottish Greens was signed in 2021.

The difficulty is that a minority ­government can no longer be sure its legislation will pass through ­Parliament.

“If the Greens do pull out there is the question of negotiating ­legislation in a parliament in which you don’t have a majority and that ­potentially becomes more difficult,” said Curtice.

“If they can’t get the Budget through there is a risk the ­Government will fall and if the Government falls, there then a risk we don’t get a new First Minister and we have to go to an election.”

As outgoing deputy First Minister John Swinney’s Budget has just been passed by the Scottish Parliament, the problem is not immediate.

However when Budget time swings round again a minority SNP ­Government would have to rely on being able to do a deal with one of the other parties in order to get the Budget through and avoid a vote of no confidence in the First Minister, whoever he or she is.

Curtice has also told The New Statesman that another ­independence referendum will happen because “the Union is in trouble”.

However, he added that getting to a ballot “may be quite a while” away.

“Half the public want out,” he said. “If you want to save the ­Union, you have to change public ­opinion, but making the case means ­explaining why Brexit is to Scotland’s ­ advantage. Good luck!”