[1] On 8 December, the constitutional council resolved to annul the result of the election in the constituency, arguing that the distribution of 10,000 and 15,000 leaflets, respectively, purporting to be from La France Insoumise and the National Front (FN) by the campaign of Boucard – without the agreement of either party – would have been sufficient to influence enough voters in the constituency to vote differently than they might otherwise would have, and potentially produced a different result, given the small vote margin separating the candidates.

[6][7] The first candidates to officially declare their candidacies were Ian Boucard for The Republicans, Christophe Grudler for the MoDem and La République En Marche!

(REM), Yves Fontanive for Lutte Ouvrière (LO), and Jonathan Vallart for the Popular Republican Union (UPR), with Jean-Raphaël Sandri invested again by the National Front.

[10] Julie Kohlenberg received the investiture of Debout la France (DLF),[11] and Sophie Montel, a Member of the European Parliament (MEP) who joined Florian Philippot in leaving the FN to form The Patriots, within which she serves as vice president, also contested the by-election.

On 18 January, Christian Jacob, president of the LR group in the National Assembly, visited the constituency in support of Boucard,[14] as did deputy Annie Genevard.

[18] At the public meeting on 18 January with Jacob and Genevard, Boucard announced he would sue Grudler for calling him a "cheater" and "fraudster" over the distribution of leaflets by LR activists purporting to be from La France Insoumise and the National Front (which resulted in the invalidation of the election by the constitutional council).

The defense lawyer stood by the characterization, emphasizing that the constitutional council ruled in favor of Grudler's challenge and invalidated the election in June.

[25] In a statement, Montel denounced Sandri's behavior and support for "authoritarian regimes", demanding that Marine Le Pen "firmly condemn" the FN candidate in the constituency.

[27][28] A debate between the two rounds was broadcast on France Bleu Belfort Montbéliard between 18:30 and 19:00 CET on 1 February, co-hosted with Philippe Piot, journalist for L'Est Républicain.