Not just, “The Camp of the Saints,” but there is a book written in 1972 about basically the same thing called, “Fugue for a Darkening Island,” by Christopher Priest, that happens in Britain instead of France.
https://en.wikipedia.org/wiki/Fugue_for_a_Darkening_Island
I just finished reading my copy. The protagonist is a Liberal, but gets mugged by reality and then becomes very conservative.
You can find it on Amazon. https://www.amazon.com/dp/0575098201
I ordered a copy of “The Camp of the Saints” back on 21 April, but it’s back ordered until the 10 June.