DALLAS (AP) — 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 Wednesday 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.

Delta Air Lines stood by a January estimate that it figures to lose $25 million in revenue from the shutdown. Other carriers have not provided estimates.

Southwest shares tumbled $2.78, or 4.8 percent, by early afternoon Wednesday. 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 order last Friday that requires mechanics to get a doctor's note if they call in sick and gives Southwest the power to impose mandatory overtime. Mechanics who don't comply could be fired.

Still, delays and cancellations from grounded planes have persisted.

Southwest had canceled about 435 flights — 11 percent of its schedule — by early afternoon Wednesday, according to FlightAware. It wasn't clear how many were related to maintenance and how many were due to winter weather that disrupted air travel in the East. Southwest canceled 200 flights Tuesday when weather was not a major factor.

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.

"That should be alarming to everybody, including management," Bret Oestreich, the union's national director, told The Associated Press recently.

Southwest and AMFA have been in contract negotiations for more than six years. Mechanics rejected a tentative agreement last fall, and union officials say the two sides remain far apart on pay and Southwest's desire to keep performing some maintenance work in El Salvador. The union says Southwest has fewer mechanics per plane than other airlines, so its workers deserve to be paid more.

Southwest has run into other safety issues. The Federal Aviation has spent a year investigating how Southwest calculates the weight of checked bags and makes sure loads are properly balanced in the cargo hold. Southwest said it reported the problem voluntarily and has made improvements.

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David Koenig can be reached at http://twitter.com/airlinewriter