The first round of a by-election was held in Val-d'Oise's 1st constituency on 28 January 2018, with a second round on 4 February because no candidate secured a majority of votes in the first round. The by-election was called due to the invalidation of the election of Isabelle Muller-Quoy, candidate of La République En Marche! (REM), in the June 2017 legislative elections by the Constitutional Council on 16 November 2017. It was the first legislative by-election held during the 15th National Assembly.

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2018 Val-d'Oise's 1st constituency by-election

← 2017 28 January 2018 (first round) 4 February 2018 (second round) 2022 →
Turnout	20.33% 27.76% (first round) 19.09% 20.51% (second round)
Nominee Antoine Savignat Isabelle Muller-Quoy Leïla Saïb Party LR LREM LFI 1st round % 3,855 23.67% 5.92% 4,768 29.28% 6.65% 1,867 11.47% 1.34% 2nd round % 7,167 51.45% 5.69% 6,762 48.55% 5.69% Eliminated   Nominee Stéphane Capdet Sandra Nguyen-Derosier Bénédicte Ariès Party FN PS EELV 1st round % 1,647 10.11% 5.19% 1,121 6.88% 1.40% 1,009 6.20% 2.33% 2nd round % Eliminated Eliminated Eliminated
Deputy before election Isabelle Muller-Quoy LREM Elected deputy Antoine Savignat LR

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In the first round on 28 January, Isabelle Muller-Quoy 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. Savignat was elected in the second round with 51.45% of the vote, defeating Muller-Quoy.

On 14 November 2017, the Constitutional Council heard an appeal regarding the 2017 legislative elections within Val-d'Oise's 1st constituency by the three main defeated candidates, Antoine Savignat of The Republicans (LR), Denise Cornet of the National Front (FN), and Leïla Saïb of La France Insoumise (FI), who argued that the election should be invalidated because Michel Alexeef, selected as a substitute to elected deputy Isabelle Muller-Quoy of La République En Marche! (REM), previously served as a president of an employment tribunal within the constituency from January 2016 to January 2017, rendering him ineligible to run in a legislative election under the electoral code.^[1]

The Constitutional Council annulled the election of Muller-Quoy on 16 November 2017 under article L.O. 132 of the electoral code, and a by-election was held in the constituency in 2018 to fill the vacant seat.^[2][3] The legislative by-election was the first to be held since the election of Emmanuel Macron, and the first to be contested by a REM candidate.^[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.^[4]

Though the poll opening and closing times were initially set as 8:00 and 18:00 CET,^[5] these hours were later extended to 20:00 CET.^[6]

On 14 December 2017, the by-election was scheduled for 28 January 2018, with a second round on 4 February should no candidate secure a majority of votes in the first round; candidates are required to file between 2 and 5 January.^[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.^[9]

On 9 January 2018, the Union of Democrats and Independents (UDI) announced its support for Savignat, whose substitute, Nathalie Groux, is an elected member of the UDI.^[10] Xavier Renou, substitute to Leïla Saïb, candidate of La France Insoumise, alleged that mayor of Beaumont-sur-Oise Nathalie Groux, substitute to Antoine Savignat of The Republicans, made it deliberately difficult to register as a candidate, a claim that Groux denied.^[11] On 13 January, Florian Philippot visited in support of Denise Cornet, followed by Christophe Castaner in support of Muller-Quoy on 14 January. Nicolas Bay visited on 19 January in support of Stéphane Capdet, and La France Insoumise deputies Alexis Corbière and Adrien Quatennens visited on 20 January to support the candidacy of Leïla Saïb, the same day that Valerie Pécresse came to support Antoine Savignat. Minister Agnès Buzyn visited to campaign with Muller-Quoy on 22 January,^[12] followed by Prime Minister Édouard Philippe on 25 January,^[13] the latter's visit considered an appeal to the traditionally right-wing tendencies of the constituency.^[14]

In the first round on 28 January, Muller-Quoy received 29.28% of the vote, compared to Savignat with 23.67% of the vote, a significantly smaller margin than that in June. Saïb arrived in third with 11.47% of the vote, followed by Capdet with 10.11%, Nguyen-Dérosier with 6.88%, and Ariès with 6.20%.^[15] On 30 January, Castaner came to Beaumont-sur-Oise to support Muller-Quoy,^[16] followed by Pécresse and Laurent Wauquiez on 31 January to back Savignat, not long after a tense meeting of the two at the national council of the party on 27 January,^[17] and were accompanied by deputy and LR vice president Damien Abad. 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.^[18] In the second round on 4 February, Savignat was elected deputy with 51.45% of the vote, defeating Muller-Quoy, who received 48.55%.^[19]