Worried about mounting resistance to genetically modified (GM) foods, the giants of biotechnology are channeling tens of millions of dollars into massive lobbying and marketing campaigns to counter their critics. Until recently, biotech companies were reluctant to respond to critics because they wanted to avoid making GM foods an issue, but now that opposition is spreading, particularly in the U.S., some company executives see the need for urgent action. Enviros have been gathering attention with anti-GM campaigns and with full-page ads in the New York Times that point out potential dangers of biotechnology. On Wednesday, a bipartisan group of 20 members of Congress introduced legislation that would require labeling of GM foods because of concerns about food safety.