AN SNP activist has explained why he wants to challenge John Swinney for the party leadership.

Graeme McCormick – a retired solicitor who has served as Convener of SNP's Dumbarton branch – was at the All Under One Banner march in Glasgow on Saturday seeking to gather the required support to stand for leader.

It is understood McCormick believes he is very likely to receive the 100 nominations from at least 20 SNP branches before the Monday deadline.

The veteran activist has previously been critical of the SNP-led Scottish Government, using last year’s party conference to criticise its independence strategy.

READ MORE: SNP activist seeking to challenge John Swinney for party leadership

McCormick previously highlighted in an exclusive interview with The National that he felt the route to independence lay in international treaties – a motion for which was rejected at last year’s conference, where he also unsuccessfully ran for SNP president.

The activist still appears to believe this route is the way forward for independence.

In a letter to The National, McCormick said he wishes to stand because “I know of no potential candidate who is prepared to call an early Holyrood election”.

“Central to the campaign is that a majority vote will result in the SNP MPs being instructed to dissolve the union, establish the sovereign government of Scotland, advise the UN that the union is ended and that Scotland will adhere to the terms of the Vienna Convention on the dissolution of a former state,” he wrote.

“Under international law, this is the "how" of delivering our nation statehood.”

The activist also took aim at the "advent of muscular unionism” since the Brexit vote in 2016.

“The powers of the Scottish parliament continue to diminish the longer the SNP is in power,” he said.

“We have reached a watershed. Managing devolution is now incompatible with prosecuting independence. As a result, our support is waning.”

The National: Graeme McCormick is running to be the SNP president

McCormick added: “The party must waste no time in debating and resolving this fundamental issue. The leadership contest is the ideal forum.

“I believe with all my heart and soul that the SNP can deliver a majority vote for an independent Scotland. The polls for Independence are consistently positive. The SNP has the opportunity to regain its lost support if it promises to dissolve the union through the Ballot Box. No other party can do so.”

If McCormick secures enough nominations and puts his name forward before a deadline of noon tomorrow then a leadership contest will take place between May 13 – 27 and potentially derail Swinney’s ascension as leader and First Minister.

Swinney, meanwhile, argued a divisive SNP leadership contest could delay the party’s essential “rebuild”.

He told the Sky News programme Sunday Morning with Trevor Phillips: “I think the SNP has got a chance to start rebuilding from the difficult period that we have had, under my leadership, and bluntly, I’d just like to get on with that as quickly as I possible can do, because every day that we spend in an internal contest, which I think we all probably know the outcome of, we delay the possibility for the SNP to start its rebuilding.”

Swinney also said his leadership bid had received “very, very comprehensive support”, including from former finance secretary Kate Forbes, who earlier ruled herself out of the contest.

If McCormick won the leadership race against Swinney, the activist would not be able to become First Minister as he is not elected in the Scottish Parliament, with the activist reportedly telling members he believes the roles should be run by two different individuals.

McCormick has been contacted for comment and more details.