In a move that reversed its long-held aversion to crowded markets, Southwest Airlines on Wednesday bid $7.5 million for the LaGuardia slots controlled by its bankrupt partner, ATA Airlines. If successful, Southwest could become a major competitor to Delta Air Lines, USAirways and American Airlines, which all have substantial operations at LaGuardia. The move is not without risks, as LaGuardia's chronic delays could increase costs and hurt the Southwest brand. But Bob Montgomery, Southwest's vice president of properties, believes those risks can be minimized. "We can effectively isolate [LaGuardia] and make sure delays don't flow down," he said.