The trade group Airlines for America said airlines’ pretax earnings stayed basically flat over the past year. But its chief economist, John Heimlich, expects 4 percent more passengers this summer.

“That will be an all-time high of 231 million passengers — 2.5 million per day on average,” he said.

And all of those passengers will have to go through security. Heimlich thinks if security delays drag on months or even years, they’ll definitely have an impact on airlines’ bottom lines.

“So it’s all the more important that we nip this in the bud now,” he said.

Airlines want the Transportation Security Administration to use more sniffer dogs and beef up staffing at busy airports. Joe Schwieterman, an aviation expert at DePaul University, said the long lines could prompt travelers to avoid short flights this summer and drive instead.

“That market’s been weak for a long time, and this just raises the hassle factor to a new level,” he said.

Schwieterman said that’s especially true for late summer travelers, who may not have booked flights yet.