The story
Probably originally a poker phrase, with "pat" equaling "optimal", meaning that drawing more cards is unnecessary. Later the phrase came to be employed in other spheres. "Stand pat" was actively used in U.S. political campaigns of the 1900s, quickly gaining a sense of "being reactionary, resistant to dramatic changes in policy". Moreover, a noun, standpatter, was coined to denote such politicians.
Etymology from Wiktionary, CC BY-SA 4.0.