In the first round of the 2017 legislative elections on 4 June, La République En Marche! candidate Samantha Cazebonne obtained a majority of votes in the first round in the fifth constituency for French residents overseas. However, she was not immediately elected because of low turnout, with candidates who secure a majority in the first round required to also obtain the votes of at least 25% of registered voters in order to avoid a second round. François Ralle-Andreoli, candidate of La France Insoumise, obtained 15.52% of the vote and advanced to the second round, while Laurence Sailliet of The Republicans, with 15.11%, was eliminated by 60 votes. After the election, Laurence Sailliet filed an appeal with the Constitutional Council. Sailliet contended that Cazebonne violated article L. 52-8 of the electoral code, which states that individuals are prohibited from supporting the campaign of a candidate by supplying "goods, services or other direct or indirect advantages at prices lower than those normally practised", and was therefore ineligible for election under article L.O. 136-1. Sailliet claimed that Cazebonne was present at a meeting of business leaders over a meal at a restaurant in Barcelona on 13 June that was financed by the France-Spain Chamber of Commerce and Industry. However, the records Cazebonne submitted to the National Commission for Campaign Accounts and Political Financing showed that the meal was paid for by each of its participants, not the France-Spain Chamber of Commerce and Industry, and this complaint was therefore dismissed. Although the complaint concerning the eligibility of Cazebonne was rejected, Sailliet also raised concerns about the administration of the election. Due to an error, voting materials were not mailed to 94 voters before the first round as required under article R. 176-4 of the electoral code; of these 94 voters, 56 did not vote despite having the right to do so. As a result of delays in the delivery of voting materials or omissions in those sent, many voters were unable to submit mail-in ballots during both rounds of voting. Although the number of voters affected by these issues was uncertain, the degree of these issues was attested to by a substantial number of voters who failed to vote, as confirmed by electoral rolls. In addition, under article L. 49 of the electoral code, diffusion of electoral messages is prohibited during the election silence. Ralle-Andreoli, who emailed a message at 20:03 local time on 3 June 2017 to some voters registered on electoral rolls within the constituency, violated this article. Given these issues and the small margin separating the second- and third-placed candidates in the first round, the constitutional council annulled the result of the election, triggering a by-election within three months. On 21 February 2018, the first round of the by-election was scheduled for 8 April 2018, with a second round on 22 April should no candidate secure a majority of votes in the first round.
