ATLANTA (Reuters) - The Association of Flight Attendants-CWA union said on Tuesday that it has withdrawn its bid to seek a union election for more than 20,000 workers at Delta Air Lines Inc, citing a proposed U.S. rule change that it said would result in a fairer balloting process.

This week, the National Mediation Board proposed a rule change under which union elections at airlines and rail companies would be decided based on the majority approval of those who vote.

The current procedure, which dates back 75 years, requires that a majority of an entire work group, whether they participate in elections or not, cast ballots in favor of unionization.

Groups representing airline companies have opposed the rule change, which could improve the odds of unions winning elections. Under the current system, workers who don't vote are effectively counted as "no" votes.

The flight attendants union had taken steps in July to seek an election at Delta, which was largely non-union until it acquired Northwest last year.

The union, which supports the proposed rule change said it would re-file its application after the National Mediation Board resolves the matter.

"We want this election at Delta Air Lines to occur under the new democratic procedures and therefore are withdrawing our single transportation application," Patricia Friend, the union's international president, said in a statement.

In a letter sent to the three-member mediation board, the flight attendants union said it would be "inappropriate and prejudicial" for the board to conduct an election for Delta until the rule change is settled.

Mike Campbell, Delta executive vice president of human resource and labor relations, said the union's action was "appalling though not surprising."

The rule change could be implemented after a 60-day comment period.

(Reporting by Karen Jacobs, editing by Leslie Gevirtz)