Fred Thomas, 31, who spent seven years in the elite commando force, will be attempting to overturn a nearly 13,000 Conservative majority in Plymouth, Moor View.
In his first national media interview, Thomas said he was partly driven to politics by his Tory opponent's claims to be helping veterans, many of whom are struggling to survive.
"I would love to see Johnny Mercer lose his seat to anyone, even if I hadn't left the Royal Marines to do this, because he uses veterans as a huge campaigning tool. He just hasn't delivered.
"Veterans are really struggling with the cost of living crisis and he is part of the government that caused that.
"While I was serving, it was incredibly frustrating, time and time again, to hear someone painting themselves as a great champion for veterans' rights when this was going on," he said.
His combative words will set up an intriguing political battle for the seat in south-west Devon, in a city that is home to many veterans, and is built around a naval base.
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