Blame Wilson

For flag lapel pins, too. Clark Stooksberry cites Kaufmann:

American flag lapel pins had been distributed to members before the president spoke to Congress on April 2, 1917, requesting a declaration of war. It took a certain obdurate courage to refuse to wear the colors; Senator La Follette was among the refusers, as was the Mississippi senator Vardaman.

Vardaman, of course, was punished accordingly.