In 1980, thirteen Members of Parliament, including Etienne Tshisekedi and Joseph Ngalula, wrote an open letter to Mobutu, stating their concerns with the authoritarian state and the evolution of the MPR after the release of the Manifesto of N'sele. Mobutu responded with repressive measures including the arrest for "aggravated treason" and incarceration of the parliamentarians. Marcel Lihau, politician and jurist, testified on their behalf at the ensuing trial. Furthermore, Mobutu underestimated the favourable response of large parts of the population to the open letter. Contrary to the 1974 constitution, the 1967 constitution provided the possibility for a second political party next to the MPR. The thirteen parliamentarians grouped together with other allies such as Lihau to form an illegal second political party, the Union for Democracy and Social Progress.
