The protest, National Opt-Out Day, is scheduled for Wednesday to coincide with the busiest travel day of the year. The Obama administration's top transportation security official implored passengers Monday not to participate, saying boycotts would only serve to "tie up people who want to go home and see their loved ones."

"Just one or two recalcitrant passengers at an airport is all it takes to cause huge delays," said Paul Ruden, a spokesman for the American Society of Travel Agents, which has warned its more than 8,000 members about delays resulting from the body-scanner boycott.