Both posters, Oniki’s and Mitsumoto’s, used recreated the Demon Slayer logo, Oniki making it into his own name, and Mitsumoto putting the party name in there.
To get the attention of their constituents, a few politicians are using various patterns and logos from Demon Slayer: Kimetsu no Yaiba to gain more notoriety on their campaign posters.
The poster is clearly using the pattern from Tanjiro’s haori to help make the poster more striking.