Robert Craig told how he would resign his post if Mr Stewart was barred from standing as a Conservative MP and warned the local party could be “devastated” by Downing Street’s hardline approach.
Speaking to the Evening Standard, he said: “Personally, I don’t appreciate central office or even No 10 basically dictating to us who we should have as our candidate for the next election when in my view we are represented perfectly ably by Rory Stewart.
“We are effectively being told by No 10 what it is to be a Conservative. I disagree with that fundamentally.”
Venting grassroots anger at Boris Johnson’s Downing Street chief adviser Dominic Cummings, who is blamed for the hardline stance against the rebels, Mr Craig added: “We have basically got an extremist advising an opportunist — that has got to be a recipe for disaster.”
He spoke out after former Tory leader Lord Hague branded the expulsion of 21 Tories including Father of the House Ken Clarke, former Chancellor Philip Hammond and Winston Churchill’s grandson Sir Nicholas Soames, as the “most egregious and counter-productive act of self-harm committed by the party leadership that I can recall in my lifetime”.
Former Foreign Office minister Sir Alan Duncan said any “self-respecting” chairmen, or chairwomen, of the 21 MPs’ local parties should have told Conservative HQ to “bugger off” when they were told their MP had been deselected.
Mr Craig, 49, president of Penrith and The Border Conservative Association and a prominent dairy farmer, made clear he would resign if Mr Stewart remains banned from standing as a Tory MP.
“If he has to fight the next election as an independent, I will be no longer involved in the Conservative association,” he said.
In a message to No 10, he added: “Don’t take grassroot support for granted. This potentially could and probably will devastate our local association. I feel less like a Conservative than I have ever felt.”
Mr Craig also stressed that the centre ground of politics was now wide open.
“It’s just bizarre that each of the two main parties is getting more extreme,” he emphasised.
He believes that whichever party “puts a moderate leader in place” could be in power for a “long, long period of time”.
Other Tory members are expected to quit over the way Mr Stewart has been treated if he is not reinstated.
“It’s desperately sad where we are at the moment,” said Mr Craig.
“It’s essential that we have people of Rory’s ability right at the heart of this matter to sort it out.
“There is a lot of talent that has been ousted — it’s not good for government and not good for the Conservative Party.
“We can’t afford to lose people of Rory’s ability from politics — it’s really short-sighted.”
Mr Craig, a director of First Milk, a major farmer-owned co-operative, told of his “frustration” over Brexit and the need to “move on”. He added: “From a business perspective it’s essential that we get it right.
“We have got to have an agreement. We should not even be contemplating leaving without an agreement.”
Writing in The Daily Telegraph, Lord Hague urged Mr Johnson to allow the 21 Tory MPs a way back into the parliamentary party, stressing: “Britain needs a strong and united party of the centre Right if it is not to become the battleground of extremism.”