For more than half a century, a committee of cultural heavyweights has met behind closed doors, its deliberations kept secret, weighing the faces and images of Americana worthy of gracing U.S. postage stamps. While its rulings have been advisory, they long carried the weight of writ.

Now comes a youngster from across the seas. He isn’t what these leading lights from the fields of arts and letters, athletics, and philately had in mind. For one, he seems kind of crass to some. And worse, he isn’t even American.

On Tuesday, the U.S. Postal Service is scheduled to release 20 postage stamps honoring Harry Potter, and officials at the cash-strapped agency hope the images, drawn straight from the Warner Bros. movies, will be the biggest blockbuster since the Elvis Presley stamp 20 years ago.

But the selection of the British boy wizard is creating a stir in the cloistered world of postage-stamp policy. The Postal Service has bypassed the panel charged with researching and recommending subjects for new stamps, and the members are rankled, not least of all because Potter is a foreigner, several members said.

The dispute caps more than a year of friction between the Postal Service and the Citizens’ Stamp Advisory Committee, named by the postmaster general to help make sure that the Amer­ican experience is properly ­portrayed. The committee has grown increasingly disaffected over how the agency’s marketing staff has pushed pop culture at the expense of images that could prove more enduring.

Set up as a filter between the postmaster general and the public, which petitions the Postal Service for about 40,000 stamp subjects and designs each year, the committee includes such eminent Americans as historian Henry Louis Gates Jr., former American Film Institute president Jean Picker Firstenberg, and Olympic swimmer and sportscaster Donna de Varona. A former postmaster general, a top Smithsonian Institution official, graphic designers and philatelists also belong.

Its mission is to ensure that stamp subjects “have stood the test of time, are consistent with public opinion and have broad national interest.”

For one of the only times in its 56-year-history, the committee was not consulted in the decision to put Potter and his friends and foes on the run of 100 million “forever” stamps.

“Harry Potter is not American. It’s foreign, and it’s so blatantly commercial it’s off the charts,” said John Hotchner, a stamp collector in Falls Church and former president of the American Philatelic Society, who served on the committee for 12 years until 2010. “The Postal Service knows what will sell, but that’s not what stamps ought to be about. Things that don’t sell so well are part of the American story.”

Postmaster General Patrick R. Don­ahoe said in an interview that the agency “needs to change its focus toward stamps that are more commercial” as a way to increase revenue to compensate for declining mail volume as Americans switch to the Internet.

Stamp sales have dropped along with first-class mail. Sales came to $7 billion last year, postal officials say.

The agency lost $5 billion last year and continues to face severe financial problems in part because of massive payments to fund health-care benefits for future retirees. It has pressed Congress for help, but a fix is unlikely this year.

Donahoe said postal officials chose Harry Potter because of subject’s appeal to young people — and in an effort to inspire that demographic to become collectors. He acknowledged that the advisory committee feels “a lot of disruption” from the agency’s change in direction. “As we move in [a commercial] direction, they’ll be working with us on that,” he said.

Members of the advisory committee have complained to Donahoe that they have been brushed aside by agency staff, led by marketing director Nagisa Manabe, a former Coca-Cola executive hired in 2012 to reinvigorate the postal brand. Manabe moved the stamp program into her department and pushed aside veterans in the program, according to postal sources.

Manabe, through a Postal Service spokesman, declined repeated requests for comment.

At her urging, the committee changed its charter to allow corporate advertising on stamps, as long as it is not featured as the main element, according to people familiar with the changes. The charter now includes “contemporary and timely” in its criteria for new stamp selection, with “educational” a factor but no longer an “essential” one.

Many ideas for new stamps now originate with her staff and are heavy with celebrity subjects, those familiar with the changes say. Among those now under consideration are the Beatles, Apple founder Steve Jobs, basketball player Wilt Chamberlain, Rudolph the Red-Nosed Reindeer and chef Julia Child.

Manabe objected to the jazz singer Sarah Vaughan, people familiar with the discussions said, on the grounds that Vaughan is not well known among young Americans. Barbie has been on the table, although no decisions have been made on the Mattel doll.

In September, the committee’s frustration boiled over and all 13 members walked out of their meeting and signed an unprecedented letter to Donahoe demanding that he meet with them.

“Quite simply, as it is run now, this committee no longer represents the collector, both avid and amateur, the child just discovering the wonder of stamps, the bride looking for the perfect wedding stamp or the very citizens it was designed to serve,” said the letter, which was obtained by The Post and first reported by Linn’s Stamp News. The committee wrote that it had responded to the mandate to “think big and think commercial.” But Harry Potter “was developed even though we, as a committee, did not propose it nor discussed how it could be best presented.”

In a response to the letter, Donahoe suggested the committee’s meetings may be cut to two a year instead of four.

While their role is being curtailed, committee members have also objected to the Manabe’s decision to hire a futurist to help map out the future of stamps. According the August statement of work, Faith Popcorn’s BrainReserve is helping to devise strategies to slow the “predictable decline in stamps use” and “reinvent and reimagine” the program.

A person familiar with the advisory committee’s thinking said the “new world order in the post office” is all about what sells. “It’s ‘Let’s not waste time or money on something that isn’t gong to be of immediate interest. National parks, scenic America won’t make the cut,’ ” said the person, who spoke on the condition of anonymity out of fear of retribution from the Postal Ser­vice.

The agency recently reissued a new $2 version of a highly publicized stamp error: a 1918 airmail stamp commemorating the country’s first airmail flight, a stamp known as the Inverted Jenny. The stamp had an upside-down image of the Curtiss JN-4 biplane used to deliver the mail.

The Postal Service reissued the inverted image in September as well as 100 sheets of the image right side up. Spokesman Roy Betts said the goal was to generate excitement.

But to committee members, as well as many collectors, it has come across as a gimmick and an unfair lottery.

The Harry Potter stamp is eliciting similar skepticism from collectors.

“The attitude should be that stamps are works of art and little pieces of history,” said Don Schilling, a collector in Los Angeles who writes a blog on the subject. “They shouldn’t be reduced to the latest fads, whatever’s going to sell.”