(March 6): In just a few days, U.S. travel has begun to crumble.

That’s the message from a sudden rash of warnings from U.S. airlines. Southwest Airlines Co. said Thursday that first-quarter sales would take a hit of as much as $300 million after a plunge in demand that Chief Executive Officer Gary Kelly called a “gut punch.” United Airlines Holdings Inc. and JetBlue Airways Corp. are planning to trim flights as they brace for a slowdown.