UPDATE 2-Machinists union seeks Delta Air Lines election

* 12,700 workers involved

* Union to also seek election for other worker groups

* Delta shares down 1 percent

(Adds company comment)

ATLANTA, Aug 13 (Reuters) - The International Association of Machinists and Aerospace Workers said on Thursday that it wants a union election at Delta Air Lines Inc (DAL.N) to resolve representation for 12,700 workers at the carrier and its Northwest subsidiary.

The union said it asked the National Mediation Board to declare that Delta and Northwest operate as a single carrier, a step in the process leading to a union election for fleet service, flight simulator and plant protection employees.

Delta bought Northwest Airlines last year.

About 5,000 of the 12,700 staff are Northwest workers represented by the machinists, said Joseph Tiberi, a machinists union spokesman. The Delta workers are not unionized.

The union also represents Northwest stock clerks, passenger service and office and clerical workers. The union plans to seek an election for them at a future date, Tiberi said.

Delta said there was no reason for more delay in resolving representation for office and clerical staff and stock clerks, a group that numbers 20,000.

"We've said since 2008 that our employees deserve to have the issue of union representation resolved quickly," Mike Campbell, Delta executive vice president of human resources and labor relations, said in a statement.

Last month, the Association of Flight Attendants-CWA union took similar action to resolve representation for more than 20,000 employees at Delta. Northwest has 7,350 flight attendants represented by the AFA, while Delta's 13,300 are not unionized, the AFA-CWA said.

Delta shares were up 1 cent to $7.21. (Reporting by Karen Jacobs; Editing by Lisa Von Ahn and Robert MacMillan)