Stephen Flynn and Alison Thewliss are set to battle it out to become the next leader of the SNP at Westminster replacing Ian Blackford.
The MP for Ross, Skye and Lochaber announced last week that he did not intend to stand again when the SNP's AGM takes place on Tuesday evening.
Thewliss, who announced her intention to stand on Saturday following a number of days of deliberation, revealed Stuart McDonald would run to be her deputy following the resignation of Kirsten Oswald.
It will be seen as a blow to Flynn as the pair were part of the so-called Tuesday Club, a group of SNP MPs who play football together, who allegedly plotted against Blackford.
While Paisley and Renfrewshire South MP Mhairi Black has been picked by Flynn to be his number two at Westminster if he is successful in the contest which he is favourite to win.
There have been divisions within the SNP group in London for some time and Flynn's decision to stand comes just two weeks after he denied rumours he would challenge for the leadership position, following speculation he was "mounting a coup" against Blackford.
One source close to Thewliss told the Record that the Glasgow Central MP actually has the momentum going into the crucial vote.
They said: "I think it's best we have a contest and not a coronation. This isn't just about having a contest for the sake of it. Alison is making a serious and robust proposal to the group. I think it's a winning one. She is getting some momentum but she still has work to do not least because for the past fortnight people have only been talking about Stephen Flynn.
"Alison is not messing about but she wants to put forward a case that is about ensuring focus and bringing the group together and taking the fight to the Tories."
According to reports one of Flynn' s supporters claimed he was becoming "lazy" and that he had the contest "in the bag".
Blackford, who took over from Angus Robertson as Westminster leader in 2017, will deliver a speech at the AGM before the result is announced.
Following his announcement last Thursday it was widely expected that Flynn would run unopposed and take his position ahead of Prime Minister's Questions on Wednesday.
Thewliss has been the MP for Glasgow Central since 2015 when she unseated Anas Sarwar, while Flynn was elected in 2019 after beating Tory Ross Thompson in 2019 in the Aberdeen South constituency.
Her supporters have been keen to emphasise her experience in dealing with the big issues at Westminster.
A source said: "She's always there at the big events, she knows what she's doing, she's good on her feet. She's got experience that she wants to put to good use in a leadership role.
"I think she brings enormous experience, a wealth of talent, good intentions, good ideas and that's what she is putting to people and asking them to get behind.
"She thinks the Westminster group should be united, I don't think anybody thinks otherwise. It should be united and collaborating within the other parts of the SNP ecosystem such as in Edinburgh, the headquarters and all the rest of it.
"Alison wants us to have a united group, the public don't vote for parties that are divided. She is determined to bring a form of leadership that helps bring people together, helps make the argument, helps take the fight to the Tories and crucially helps people how to win.
"She will build a team that gives us a sense of purpose and refocuses us, which I think we were due anyway post the Supreme Court. In a sense it is all very timely, but she is focused on bringing unity to the group."
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