“Why can’t we focus on the positive things in our history?” she asked.

“It is a positive thing to deal honestly with history,” her husband insists.

Since then, common ground has been stripped to the essence: An assumption of good will and rationality. And a focus on things they share — above all the well-being of their three sons. They have a rule: Neither parent is allowed to take the children on political marches.

Sometimes — rarely — one side learns from the other.

Mr. Lethen says that perhaps liberals like him have been naïve at times.

“In 1968, our love of the foreign exploded the ring of a community of elites — we could identify with the Viet Cong, we listened to African beats, we welcomed other cultures as an enrichment,” he recalled. “It never occurred to us that these foreign lovers could turn on us or have certain values that are incompatible with ours, like the separation of church and state.”

Ms. Sommerfeld offers that she is appalled at the views of some far-right speakers “who sound like they want to purge anyone with leftist views.”

“That is really ugly bigotry on the part of the right,” she said.

Where there is a right there needs to be a left, she added.

“We are tied to one another, for better or for worse,” Ms. Sommerfeld said, as she sipped the herbal tea her husband had just brewed to soothe her sore throat.

It was not clear whether she was speaking about her marriage or her country. Or both.