President Trump has appeared on camera several times to support victims of Hurricane Harvey.

What has also received air time? The "Official USA 45th Presidential Hat," for sale by Trump's campaign for $40.

The watchdog group Citizens for Responsibility and Ethics in Washington, or CREW, points out that the president has been pictured wearing the hat, both in red and in white, in two official photos released by the White House. He wore the white version on his trip to Houston Tuesday.

"Even removed from the hurricane, it's pretty inappropriate," CREW spokesman Jordan Libowitz said.

The issue at play here is free advertising for Trump products, according to CREW.

As @realDonaldTrump leaves for Texas, he makes sure he's carrying the hats he's been advertising--yours for just $40 on his campaign website pic.twitter.com/jCtexY3iOx — Citizens for Ethics (@CREWcrew) August 29, 2017

Libowitz said he does not believe the president is violating any ethics rules by wearing the hat.

"There is a difference between what is technically legal and what is ethically right," he said.

The White House did not respond to a request for comment.

Ethics experts have argued that the president's heavily photographed weekend visits to his golf courses and Mar-a-Lago club amount to gratuitous marketing.

Trump refused to sell his business holdings as president. Instead, he transferred them into a trust in his name. Any business profits will ultimately go to him when he leaves office.

CREW is suing President Trump over his business interests. It claims he is violating a section of the Constitution that bars officials from accepting anything of value from a foreign government.

Related: Trump can't stop advertising his own brand

Trump sported the white version of the USA cap in an official photo distributed by the White House on Saturday after his teleconference with cabinet members about Harvey.

He was pictured wearing the hat in red in a photo distributed by his press office on Sunday ...

... And he kept one on for his trip to Texas, where he surveyed damage from Harvey and met with local leaders.