Last Tuesday, the Department of Justice and six state attorneys general filed suit to block the planned merger of American Airlines and US Airways. I criticized the lawsuit here on several grounds. One of the more interesting aspects of this case is who is not supporting the Obama administration: organized labor. Once the news broke, the union representing US Airways flight attendants described the Obama administration's antitrust lawsuit as based in "fantasy" and a "war on workers," joining the union representing American's pilots in blasting the attack on the planned merger. Yesterday, the union representing American's flight attendants began targeting the six attorneys general, calling on them to "reconsider [their] participation in this ill-advised lawsuit," while American's pilots union took out a full-page ad in the the Dallas Morning News urging Attorney General Greg Abbott to drop his opposition to the merger. Today, the AFL-CIO's Transportation Trades Department -- essentially the U.S. labor movement's umbrella organization for transportation unions -- wrote a letter to U.S. Attorney General Eric Holder, saying, "Your actions run counter to the interests of employees at these two air carriers represented by TTD unions and are inconsistent with the DOJ’s recent treatment of transactions involving airline consolidation." The Obama administration's misguided antitrust attack risks alienating his most important constituency: organized labor. After public-sector unions, transportation unions spend the most on lobbying and political expenditures within the labor movement. As more groups line up against the move to block the American-US Air merger, it is becoming increasingly apparent that the president bit off more than he can chew.