So I decided to check on a whim how the cities that went to FN voted for the first round
Beziers (75 000 inhabitants, biggest city with a FN mayor)
2014 Municipales
1st round
M. Robert MENARD Liste Front National - 44.88 %
M. Elie ABOUD - 30.16 %
M. Jean-Michel DU PLAA Liste Union de la Gauche - 18.65 %
M. Aimé COUQUET Liste du Front de Gauche - 6.29 %
2nd round
M. Robert MENARD Liste Front National - 46.98 %
M. Elie ABOUD - 34.62 %
M. Jean-Michel DU PLAA Liste Divers gauche - 18.38 %
2017
LE PEN 31.23 %
MELENCHON 20.28 %
FILLON 19.11 %
MACRON 17.56 %
HAMON 4.85 %
DUPONT-AIGNAN 3.45 %
LASSALLE 1.35 %
ASSELINEAU 0.88 %
POUTOU 0.74 %
ARTHAUD 0.39 %
CHEMINADE 0.16 %
I think it's pretty clear here that in 3 years the FN might be down but even if that's the case not by much.
Le Pen still leads and Macron came fourth, here more than elsewhere a biggest chunk of Melenchon and Fillons electors will probably go for her, if you add Asselineau and a good part of Dupont-Aignant she will easily be above 40% and might go up to 45 or even 50%.
Fréjus (53 000 inhabitantsr)
2014 Municipales
1st round
M. David RACHLINE Liste Front National - 40.3 %
M. Philippe MOUGIN Liste Union de la Droite - 18.85 %
M. Elie BRUN Liste Divers droite - 17.6 %
Mme Elsa DI MEO Liste du Parti Socialiste - 15.58 %
M. Philippe MICHEL Liste Divers droite - 7.65 %
2nd round
M. David RACHLINE Liste Front National - 45.55 %
M. Philippe MOUGIN Liste Union de la Droite - 30.43 %
M. Elie BRUN Liste Divers droite - 24.01 %
2017
LE PEN 33,50 %
FILLON 25,87 %
MACRON 17,14 %
MELENCHON 12,64 %
DUPONT-AIGNAN 4,73 %
HAMON 3,07 %
ASSELINEAU 1,06 %
LASSALLE 0,81 %
POUTOU 0,67 %
ARTHAUD 0,40 %
CHEMINADE 0,11 %
That's city is rightist alright, Le Pen most likely has a good shot of coming ahead of Macron here as he clearly isn't Fillon.
Hénin-Beaumont (26 000 inhabitants)
2014 Municipales
M. Steeve BRIOIS Liste Front Nationa 50.25 %
M. Eugène BINAISSE Liste Union de la Gauche 32.04 %
M. Gérard DALONGEVILLE Liste Divers gauche 9.76 %
M. Georges BOUQUILLON Liste Divers gauche 4.05 %
M. Jean-Marc LEGRAND Liste Divers droite 3.88 %
2017
LE PEN 46,50 %
MELENCHON 19,07 %
MACRON 14,86 %
FILLON 8,41 %
HAMON 4,47 %
DUPONT-AIGNAN 3,27 %
POUTOU 1,01 %
ARTHAUD 0,91 %
ASSELINEAU 0,82 %
LASSALLE 0,55 %
CHEMINADE 0,13 %
3 years in power do not seem to have weaken the FN much here either, their score are pretty similar considering the wider offer there was for the presidential election.
Villers-Cotterêts (11 000 inhabitants)
2014 Municipales
1st round
M. Franck BRIFFAUT Liste Front National - 32.04 %
M. Jean-Claude PRUSKI Liste Divers - 22.15 %
M. Jean-Claude GERVAIS Liste Divers droite - 13.61 %
M. Yves RICHARD Liste Divers droite - 13.41 %
M. Fabrice DUFOUR Liste Divers - 12.63 %
M. Jean-Claude MONNIER Liste du Front de Gauche - 6.13 %
2nd round
M. Franck BRIFFAUT Liste Front National - 41.53 %
M. Jean-Claude PRUSKI Liste Divers - 34.66 %
M. Jean-Claude GERVAIS Liste Divers droite - 23.8 %
2017
LE PEN 34,26 %
MELENCHON 19,95 %
MACRON 18,34 %
FILLON 14,33 %
HAMON 4,89 %
DUPONT-AIGNAN 4,61 %
ARTHAUD 1,04 %
POUTOU 0,88 %
LASSALLE 0,84 %
ASSELINEAU 0,59 %
CHEMINADE 0,27 %
One could say that here the FN score even improved since 2014, Le Pen will definitely be above 40% and has a good shot of beating Macron too
There are others notable cities (11 with >10 000 inhabitants) but in the end I think we notice that being in power didn't weaken the FN influence at all there, it's either stable, slightly above or slightly below compared to the previous election.
Mind I can't analyze exactly how are these cities being handled but it seems like the mayor didn't burn the FN electors yet.