Bryan was a national figure for thirty years before Dayton. He took the Democratic nomination in 1896 at thirty six on the strength of a single speech against the gold standard, ran three times and lost three times, served as Secretary of State, and resigned in 1915 rather than sign a note to Germany he thought would bring the country into the war. He campaigned for prohibition, for the direct election of senators, for a federal income tax and for women's suffrage, and he was the most effective popular orator in American public life.
His campaign against the teaching of evolution belongs to the last five years of that career, and his own stated reasons are more specific than the caricature. He had read the war literature arguing that German militarism drew on a struggle-for-existence ethic, and he had read the American eugenics movement, which by the 1920s was in the biology textbooks he objected to, including the one used in Dayton, which classified humans into races and recommended preventing the unfit from reproducing. He treated evolution as the scientific warrant for a politics he had spent his life opposing.
His second argument was procedural and democratic. Parents and taxpayers, he held, are entitled to decide what is taught in schools they pay for, and a teacher paid from public funds is not free to teach against their wishes. He was not, on the record, a young earth creationist; under examination at Dayton he allowed that the days of creation might have been long periods.