The Association of Flight Attendants on Tuesday made good on its threat to file interference charges alleging that Delta Air Lines engaged in unlawful conduct during the recent unsuccessful union election for the airline's 21,000 flight attendants.

The union said it asked the National Mediation Board to investigate what it calls Delta's anti-union literature, excessive supervisor surveillance and meddling, repeated supervisor phone calls to employees' homes and its encouragement of voting on company computers, potentially a breach of voting secrecy. If interference is found, the board can order a new election.

In a statement, Delta said the union filing was disappointing because it "prevents us from moving forward with our integration and from aligning pay, benefits and work rules" after the 2008 Northwest Airlines merger. Delta earlier called the interference claim "ridiculous" and said it will file a formal reply.