Specifically, how did they make the leap from underwear to logo'd advertisements and billboards for a persons political viewpoints, etc?
There's a somewhat related post from /u/catalot here that dips into it: https://www.reddit.com/r/AskHistorians/comments/5xirgx/when_did_writing_on_shirts_become_a_thing/
Some more from /u/eufonie: https://www.reddit.com/r/AskHistorians/comments/3odn88/what_was_the_first_band_shirt/
And while not the 'hardest' of sources, this article includes some historical photos of tshirts: https://www.mnn.com/lifestyle/natural-beauty-fashion/stories/13-iconic-moments-in-the-history-of-the-t-shirt