His claim to the presidency following the assassination of President Jovenel Moïse is disputed, due to a lack of constitutional guidance and the fact that only ten elected senators remain in the legislative body. Acting Prime Minister Claude Joseph has also claimed to take charge of Haiti following the presidential vacancy in 2021. The United Nations recognizes Joseph as the legitimate Acting President. On 9 July, a group of eight senators of Haiti nominated Lambert to replace assassinated Jovenel Moïse as president, yet this move has not been recognized by the acting prime minister Claude Joseph.