The nightmare vision focuses on a stark, painful moral choice.

It's Election Day. A Catholic voter who embraces her church's Catechism, or an evangelical committed to ancient doctrines on a spectrum of right-to-life issues, steps into a voting booth. This voter is concerned about the social impact of gambling, attempts at immigration reform, a culture fractured by divorce, battles over religious liberty and the future of the Supreme Court.

In this booth the choice is between Donald Trump and Hillary Clinton. Period.

"That's the scenario people I know are talking about and arguing about," said Stephen P. White of the Ethics and Public Policy Center in Washington, D.C., author of the book "Red, White, Blue and Catholic."

Many religious conservatives believe they "face a choice between two morally repugnant candidates," he added. "The reality of that choice is starting to drive some people into despair. … I understand that, but I think it would be wrong for people to think that they need to abandon politics simply because they are disgusted with this election."

This nightmare for religious conservatives is especially important since, in recent decades, successful Republican presidential candidates have depended on heavy turnouts among white evangelical Protestant voters and on winning, at the very least, a majority of "swing votes" among Catholics who frequently attend Mass.

While this year's election is in some ways unique, traditional Catholics and other moral conservatives need to realize that they are engaged in a debate that has been going on for centuries, said White. The big question: "Can Christians be good citizens?"