MEP Alyn Smith is facing a deselection threat after infuriating party members during the debate on Nato at the SNP conference, the Sunday Herald understands.
Smith was booed and jeered after calling opposition to the nuclear alliance "hopelessly naive" and a sign the SNP was "not ready for the big league".
Describing the party's 30-year-old policy of Nato withdrawal as "not fit for purpose", he told delegates in Perth: "In my considered, regretful, professional view, our present policy on Nato is ... hopelessly naive and idealistic at best, not ready for the big league."
The off-the-cuff remarks prompted a plot to stop Smith being returned to Brussels in the 2014 European elections, with one critic saying his "arrogance" had to be stopped.
One of two SNP MEPs, Smith, 39, would need to rank at least second on the party's candidate list next spring to have a real chance of being elected for a third five-year term.
The backlash is a sign the debate on Nato, which the SNP leadership won by just 29 votes, may yet have a bitter legacy.
Angus Robertson, the SNP defence spokesman who proposed joining Nato, is also seen as having been damaged by the issue.
"There was a time Smith and Robertson were being talked about as potential leaders. That's over," said one SNP source.
A source close to Smith discounted any plot as doomed to fail.
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