In 1932, he testified as a witness in the Brest trials, lasting from 26 October 1931 to 13 January 1932, held at the Warsaw Regional Court where leaders of the Centrolew, a "Centre-Left" anti-Sanacja-government political-opposition movement, were tried. In 1937 Bartel was appointed Senator of Poland by the President (replacing the deceased Emil Bobrowski) and served until the outbreak of World War II. In the autumn of 1938, he was one of the signatories of a document addressed to the President Mościcki, which called for the inclusion of representatives of the opposition to the government in connection with the threat of the country's independence. This document also postulated amnesty for politicians of the opposition, who were forced into exile or were imprisoned after the Brest trials. Bartel handed over a memorandum to Mościcki, however, Mościcki did not respond to the proposals contained therein. In February 1939 Bartel delivered a speech in the Senate, which has gained wide publicity in the country. In it he sharply criticized the situation in universities and colleges around Poland, mentioning the widespread anti-Semitism there, and also the failed organization of studies, subjects and courses.