Whilst watching the French News on France 2, I heard the expression “Lame de fond” on the topic of the French Presidential Elections.
Jean Marie Le Pen décèle un mouvement de fond, une lame de fond dans l’electorat.
The expression “un mouvement de fond” can be translated as:
A grassroots movement
The expression “une lame de fond” can be translated as:
A groundswell
Hence the whole phrase can be translated as:
Jean Marie Le Pen detects a grassroots movement, a groundswell in the (French) electorate.