Is Joe Manchin holding democracy hostage? His colleagues won’t say.

Increasingly, the ideal of a republic that puts some guardrails on popular passions is being criticized by those who feel that a majority-wins system would be fairer. How much power should one lawmaker have?

Inside the air-conditioned halls of the august U.S. Capitol, reporters and lawmakers alike are jockeying for a word with Sen. Joe Manchin of West Virginia, the fiscally conservative Democrat who is threatening to hold up his party’s $3.5 trillion signature budget bill.

Outside, in the sweltering September sun, protester Cathy Hook is demanding he answer a...