The opponents, led by Expedia Inc., have formed a coalition called FairSearch.org to fight Google Inc.'s proposed $700 million acquisition of ITA Software. Google's deal was announced nearly four months ago.
FairSearch argues Google would be able to stifle competition and drive up air fares if it gains controls of ITA, whose software plugs into the websites of most major U.S. airlines.
Google contends ITA would allow it to build better travel tools that help both consumers and the travel industry.
The U.S. Department of Justice already has been investigating the deal.