There are various strands to Dr Lisa Cameron’s defection to the Conservatives. 

The three-term MP for East Kilbride, Strathaven and Lesmahagow quit a few hours before she would have been deselected as the SNP’s general election candidate.

Her rival for the position, HQ staffer Grant Costello, was cruising to victory in the vote of local members before Dr Cameron’s exit meant he won by default.

Humza Yousaf claims “she probably never believed in [independence] in the first place”, suggesting she was motivated by looming defeat and self-preservation.

SNP sources suggest her reward for betrayal may be a seat in the Lords after the election courtesy of Rishi Sunak’s resignation honours list. 

Significantly, she arranged the switch directly with the Prime Minister and No10. 

Dr Cameron’s explanation referred to being a victim of the “toxic and bullying SNP Westminster group”, a culture previously called out by MP Joanna Cherry KC.

But Dr Cameron conspicuously failed to address her imminent deselection, leaving former colleagues in the SNP to fill in the blanks with gusto.

One senior source pinpointed January 17 as the day her future in the SNP ran out. 

That was when she wrote a letter to Scottish Secretary Alister Jack urging him to “intervene” to address concerns about Holyrood’s Gender Recognition Reform Bill.

Coming 24 hours after Mr Jack vetoed the Bill, it was seen as an endorsement of his action, even egging him on in the teeth of SNP fury.

READ MORE: SNP MP Lisa Cameron defects to the Conservatives

One senior SNP source said: “Lisa wrote a letter demanding a Tory Secretary of State over-rule the Scottish Parliament. That does not work for our members. 

“Her fate was sealed the moment she did that. That’s what caused her to lose the selection process. She’s the agent of her own downfall.”

However Dr Cameron’s exit was not purely about the deselection threat.

The Herald understands she had been unhappy for at least 18 months, confiding in former Scottish Secretary David Mundell, who is her local MP and represents the seat next to hers. He told her last year that she would be “welcome” if she crossed the floor.

A Tory source said that like many Yes campaigners elected in 2015, she was resented by some long-serving activists in her  local party as a queue jumper.

READ MORE: SNP set to change independence strategy yet again at conference

She was then perceived as being close to Alex Salmond, which “put her on the back foot” when he and Nicola Sturgeon had their spectacular falling out in 2018.

Her social conservatism - the pro-Life MP opposed banning protests outside English abortion clinics in 2020, for example - further alienated Team Sturgeon.

“She was given a really, really hard time,” the source said.

“Joanna Cherry got that too, but Lisa doesn’t have the same hard shell. It’s clear she’s been wrestling with these issues for some time and been treated appallingly within the SNP.”

Dr Cameron is not expected to stand for re-election, lending weight to the Lords theory.  

One of her critics in the SNP said she was a drama-seeking "narcissist" who would love the status of a peerage.

READ MORE: 'Divided parties don't win elections', Humza Yousaf warns SNP

But even they admitted she had been on the side of the angels in standing up for the young SNP staff member who was the victim of sex pest SNP MP Patrick Grady.

They also said she was highly intelligent and an inveterate hoarder of information who would be able to repay the Tories with years of knowledge about the SNP’s internal operations.

Until election day, she will “cause as much mayhem and trouble as possible” for her old party, the source predicted. 

Mr Yousaf may regret being so dismissive of her.