In an attempt to outnumber Republicans at the polls in November, union officials are getting an early start with voter registration drives in their plants. Successful efforts to curb collective bargaining rights in neighboring states like Wisconsin and Indiana are adding to a sense of urgency already heightened by the steep decline in ranks. The union has about 400,000 members — less than a third of its size 30 years ago.

Mr. King has told local chapter officials they should aim to have conversations with every member they oversee about the importance of voting in this election. Other officials are busy raising money for the U.A.W.’s political fund.

And a new online organizing network for the union, called Gimme Five, is constantly adding new members, who are notified by text message and e-mail about events like Friday’s protest, a joint effort planned with a local Democratic Party group.

“We plan to be very active in this election,” Mr. King said in an interview. “We’re building a broad coalition that we hope will help President Obama get re-elected.”