The by-election was called due to the invalidation of the election of Isabelle Muller-Quoy, candidate of La République En Marche!

and Antoine Savignat of The Republicans (LR) advanced to the second round, as they did in June 2017, with Muller-Quoy securing a considerably lower share of the vote amid low turnout.

[4] Since the 2017 legislative elections, the Constitutional Council received 298 appeals within 122 constituencies; since 21 July, 242 of these were rejected, with another forty cases outstanding when the result in Val-d'Oise was annulled.

[7] As of 17 December 2017, Isabelle Muller-Quoy declared her candidacy for La République En Marche!, Antoine Savignat for The Republicans, Stéphane Capdet for the National Front, Leïla Saïb for La France Insoumise, Sandra Nguyen-Derosier for the Socialist Party (PS), Bénédicte Ariès for Europe Ecology – The Greens (EELV), Hélène Halbin for Lutte Ouvrière (LO), and Denise Cornet for The Patriots (LP), having stood under the FN label in June.

[8] In addition, Huguette François of the Party of France (PDF), previously invested in June as part of a far-right electoral alliance, Debout la France (DLF) candidate Jean-Paul Nowak, Christophe Hayes of the Popular Republican Union (UPR), and Brigitte Poli of the French Communist Party (PCF) also filed to run.

On 1 February, Philippe and Castaner visited the constituency together at a public meeting in support of Muller-Quoy, who was introduced by deputy Aurélien Taché.