DALLAS >> Southwest Airlines is lashing out at the union representing its mechanics, suggesting that they are purposely grounding planes in order to gain leverage in new contract negotiations.

Separately, Southwest said today that the partial shutdown of the federal government will cost it $60 million in lost revenue during the first quarter — far more than the airline’s previous estimate of a $10 million to $15 million.

Southwest said it has continued to see softer bookings that it blames on the shutdown, which ended officially on Jan. 25. The earlier estimate covered the period through Jan. 23.

Southwest shares tumbled $3.12, or 5.4 percent, by midday today. Shares of several other U.S. airlines were down about 1 percent.

On the labor front, Southwest is fighting the Aircraft Mechanics Fraternal Association, which represents nearly 2,400 Southwest mechanics.

Chief Operating Officer Mike Van de Ven said Southwest saw an increase in aircraft being declared out of service on Feb. 12, “just days after our last negotiations session with AMFA.” The surge, concentrated at four maintenance shops, occurred even though there were no changes in the maintenance programs, he said.

The airline issued an emergency all-hands order to the union last week, but delays and cancellations from grounded planes have persisted — even worsened.

Southwest had canceled more than 400 flights — 10 percent of its schedule — by late morning Wednesday.

Van de Ven said “AMFA has a history of work disruptions” — Southwest has two pending lawsuits against the union — and the airline is considering all options to fix its operation.

The union counted that Southwest is “scapegoating” mechanics, and it warned that the conflict “does not bode well” for safety at one of the nation’s biggest airlines.

“For Southwest’s leadership to connect the airline’s self-declared ‘operational emergency’ to collective bargaining negotiations is simply an attempt to divert attention away from the airline’s safety issues,” the union said in an unsigned statement.

AMFA accuses Southwest of pressuring mechanics to approve planes for service too quickly because planes that are grounded do not make money for the airline.