Final answer:
The text does not directly state which U.S. president argued that abandoning the field of religious discourse in politics was a mistake. However, considering the role of religion and progressive reforms in politics, Woodrow Wilson emerges as a potential figure who supported state-level progressive reforms while believing in limited federal intervention.
Step-by-step explanation:
The U.S. president who, according to the given information, may have implied that it was a mistake for progressives to 'abandon the field of religious discourse' in politics is not explicitly mentioned. However, within the provided excerpts from LibreTexts™, presidents from different eras are discussed including Ulysses S. Grant, Woodrow Wilson, Franklin Pierce, Theodore Roosevelt, William Howard Taft, Henry Wallace, and others. Among these figures, Woodrow Wilson stands out for having strong opinions on the role of the federal government and progressive reforms. Although he remained supportive of progressive reforms at the state level, Wilson believed that the federal government should refrain from intervening too much, indicating that he could potentially view the abandonment of religious discourse as an error, should religion contribute to local control and governance.
Nevertheless, the question suggests that progressive abandonment of religious discourse might be considered a mistake from a political standpoint. The historical context provided discusses the various presidents' stances on progressive reforms, political ideologies, and the impact of these on the political landscape of their times. This indicates a nuanced interaction between religious beliefs, progressive movements, and political policy which requires critical thinking and analysis to fully understand undercurrents of U.S. political history.