One historical counterexample of the French right considering the far left a greater danger than the far right is the Resistance against the Nazi occupation, which despite mutual suspicions comprised everything from communists to right-wing republicans.
Hence Jean Moulin and Guy Môquet.
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Ditto the Italian Partisans and the post-war parties. The old right which fought the Fascists (Christian Democrats) has now largely been absorbed by the Partito Democratico or the small breakaway centre parties - they initially all joined forces against Berlusconi under Prodi.
At the grassroots, there was a nationalist factor driven by the humiliation of surrender and the indignity of foreign occupation. And at the leadership level it was a decision driven by the exigencies of the moment.
But the sense of solidarity made a strong imprint on postwar French politics.