WARREN (AP) — Vivian Johnson proudly wore her United Auto Workers jacket to hear Democratic presidential hopeful Barack Obama speak on Wednesday.

She was one of the only people outwardly representing one of the largest unions in North America, even as Obama outlined his hopes and plans for the revival of the troubled domestic auto industry. And even as many proudly displayed their allegiance to the Teamsters, United Food and Commercial Workers, and other labor groups that have endorsed Obama.

Johnson walked into the town hall at Macomb Community College north of Detroit uncommitted — just as the UAW has not endorsed either Obama or his rival, Sen. Hillary Rodham Clinton. But unlike her national union, she left with her mind made up.

‘‘I’m off the fence now, so to speak,’’ said Johnson, 42, of Local 372, which represents workers at Chrysler LLC’s Trenton engine plant. She’s the chair of the local’s Community Action Program, which handles the union’s political act