BATON ROUGE, La. — John Bel Edwards was sworn in the Deep South’s only Democratic governor this week, and already he has experienced momentous triumph — and momentous defeat.

In his first major act in office, Mr. Edwards signed an order expanding Medicaid under the Affordable Care Act, extending health benefits to roughly 300,000 poor Louisianans. Some of his supporters cried as he signed it.

But he also watched helplessly as the state’s Republican-controlled House rejected his handpicked candidate for the powerful position of speaker, a post that, by curious unwritten custom, the Louisiana governor has long had the right to select.

“I think he thought he was going to push us around,” said Representative Alan Seabaugh, a Republican from Shreveport. “He found out today, very clearly, that he can’t do that.”