Harkin was reacting to a vote in the Michigan legislature, where that body approved a right-to-work law earlier this week that makes paying union dues optional in the state.

Harkin said Michigan workers were victims of “back room political trickery” because the law was passed in a way that doesn't allow a ballot referendum to repeal the measure.

Harkin said middle-class workers who haven’t even been a member of a union have benefited from union negotiations, citing examples such as the 40-hour workweek, over-time and worker safety measures.

“I feel sorry for those workers who were caught off guard in Michigan,” Harkin said. “I feel sorry for the middle class in Michigan.”

Harkin said the American economy needs a strong middle class in order to grow. Republicans argue that unions make it hard for Americans businesses to compete globally.