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Polls indicate that Mr. Melenchon has the support of about 15 percent of the French electorate, which would knock him out in the first round of voting, leaving Mr. Hollande to go head-to-head against Mr. Sarkozy in the runoff vote.

If Mr. Hollande wins by a narrow margin with votes from Melenchon supporters, some say, Europe’s conservatives might have a real reason to worry.

“If [Mr. Melenchon] is seen as kingmaker, if he and his supporters are getting serious Cabinet posts in an administration headed by a President Hollande, then I think you see the prospect of something much more dramatic potentially happening,” Mr. Tonra said.

“Clearly, the agenda of Melenchon and his supporters is much more radical, much more antithetical to the entire thrust of where EU policy has been for the last couple of years. And that would set off fireworks.”