Kammala jumla mai zuwa: Da yake zaune a Chicago, Mosteller ya taimaka tare da yakin neman zaben shugaban kasa na Obama a farkonsa a watan Fabrairun 2007. Yin aiki a Iowa, New Hampshire, da sauran jihohi masu mahimmanci, ya haɓaka yawancin manufofi da hanyoyin da ƙungiyoyin ke amfani da su don shirya tafiye-tafiye da abubuwan da ɗan takarar ya yi. Ya shiga cikin kungiyar da ke wurin muhawarar farko ta 2008 kuma ya aiwatar da rawar sanata a
Taron kasa na jam'iyyar Democrat a Denver. Bayan zaben Obama, Mosteller ya koma Washington, DC kuma ya fara aiki a Ofishin Oval bayan sa'o'i bayan kaddamarwa. Mosteller yana daya daga cikin ma'aikatan da suka fi dadewa a cikin gwamnati. Abokan aikin da ba a ambata sunansu ba sun lura cewa "idan kana son [saduwa] da ƙarfe biyu na dare, sai ka tuntuɓi shi". A cikin hira ta 2017, Obama ya ce Mosteller, "Manaja ne mara gajiya kuma jagora tare da ƙwarewa don yin gumi da ƙananan abubuwa". A cikin "West Winging It", wanda aka buga a cikin 2018, marubucin ya ambace shi a matsayin "mai alhakin kiyaye mutunci a duk faɗin West Wing".