Watch Mama Cass tell John Denver about voter registration before joining him to sing "Leaving on a Jet Plane" in 1972

In this 1969 episode of Midnight Special, Cass Elliot talked to John Denver about her work getting college students to register to vote. "This is very important," Elliot said. "I've been traveling around the country for the past year or so, talking on a lot of college campuses, trying to find out exactly what people are thinking, and the thing that's impressed me the most is that there is, still in this country, believe it or not, after all the talk, a tremendous amount of apathy on the part of people who maybe don't like the way things are going and maybe want to change it but don't do anything about it."

Elliot then joins Denver in singing his song, "Leaving on a Jet Plane."