It is also mentioned in chapter 6 of Visions of Glory by Barbara Grizzutti Harrison:
He practiced trial law in Boonville, Missouri, for fifteen years, campaigning briefly for William Jennings Bryan.
I haven't been able to discover where they got that bit of information from.
A.H. MacMillan briefly mentions WJB in Faith on the March:
William Jennings Bryan, as Secretary of State to President Woodrow Wilson, had made a tour of the country urging the United States of America to stay out of World War I, while most of the clergy were urging the government to get into it. Bryan resigned when, April 6, 1917, the United States entered the war.~Merry