Earlier this month, we published an article on Mexican towns and cities that are dealing with public corruption and record violence by effectively seceding from the state. But we wanted to elaborate more on this story, to recount what it was like to report on each town, which we visited last year, and offer our thoughts on its larger lessons for Mexico and the world.

This is the second of a three-part series that originally appeared in our Interpreter newsletter. The third will publish later this month. So sign up the Interpreter newsletter here, and let us know what you think: interpreter@nytimes.com.

When we talk to political scientists who study American democracy about what worries them most — and they worry about many things these days — their answer is often political parties.

Whatever you think of President Trump, they argue, he has exposed the frailty of the Republican Party, which tried and failed to stop his nomination. The party has since bent on core issues, especially those related to the Russia investigation. Both the Republican and the Democratic parties, scholars argue, have grown dangerously weak, unable to play their informal but crucial role as institutional checks on power; as guardrails of democracy.