When Michelle Obama steps out at Monday night's inaugural galas, life will change for at least one person who is unlikely to be in the room: her gown's designer. Accolades will flow, if the past is any guide, and morning television shows will race for the first interview. The designer's current collection should quickly sell out.

Despite a strict policy against commenting on her clothes, Mrs. Obama has become her generation's most impactful fashion muse. Already, the fashion industry is trying to horn in on the spotlight she will generate next week.

Retailer White House | Black Market this month introduced a limited-edition "inaugural-inspired" dress collection of nine beaded gowns and cocktail-length numbers—priced accessibly between $250 and $300. Aiming to grab some publicity from Mrs. Obama's jewel choices, the Gemological Institute of America speculated that she would wear emeralds at the inauguration because color experts Pantone declared emerald to be Color of the Year for 2013.

The fashion world is abuzz with gossip about which designers Mrs. Obama will wear on Monday. Naeem Kahn is being mentioned as a contender for her gown. Jimmy Choo is considered a shoe-in for her heels.

Consumers paid little attention to Laura Bush's steady diet of Oscar de la Renta suits, and few emulated Hillary Clinton's tailored-pantsuit wardrobe. Those choices fit the women's roles as Washington, D.C., figures and were priced beyond reach for most people.

But Mrs. Obama wore a Gap sweater to lunch with Nancy Reagan and also introduced the world to Jason Wu's $1,500 dresses, influencing the way women dress at work. Her risk-taking has led many women to incorporate more feminine dresses and blouses into their power wardrobes and to don soft cardigans in place of traditional suit jackets at the office.

Most first ladies "fall into the trap of dressing to fit in," says Kate Betts, a fashion editor who wrote one of a bevy of books on Mrs. Obama's fashions, titled "Everyday Icon: Michelle Obama and the Power of Style." "She took the opposite tack: She dressed to stand out."

Entire brands—such as J. Crew, which Mrs. Obama wore on David Letterman's show—have soared on the draft from her patronage. "She's almost to the point of launching collections," says fashion-industry investment consultant Robert Burke, noting that what Mrs. Obama wears sells out, even if it isn't widely available for weeks or months later.

Political kerfuffles have broken out over some of her fashion decisions, often because she breaks with traditional symbolism. Many people were outraged at her decision to wear a non-American label—the British Alexander McQueen's full red gown—to a state dinner for the Chinese president in early 2011. And she made headlines by wearing a cardigan to meet Queen Elizabeth II. Though the cardigan was by Paris couturier Azzedine Alaia, many deemed the choice too casual for a royal moment.

Amid her frequent donning of down-to-earth Gap and J. Crew clothes, critics found fault when Mrs. Obama once wore Lanvin sneakers—priced at an estimated $540—to a food bank.

Mrs. Obama supersedes even A-list celebrities and the future queen of England in her fashion impact, Mr. Burke says. "Kate Middleton has global influence, but she's not as fashionable," he says. And because Mrs. Obama cultivates an every-woman persona, he says, "Unlike Lady Gaga, women can relate to her."

Fashion executives have taken to including photos of Mrs. Obama wearing their brands' clothing when they prepare investment packages for bankers and private-equity investors, Mr. Burke says.

The first lady's patronage doesn't necessarily lead to stardom. Chicago designer Maria Pinto went out of business despite a long association with Mrs. Obama. Isabel Toledo, who designed Mrs. Obama's yellow coat and dress for the last inaugural swearing-in, isn't much more famous now than she was before January 2009.

But if you know who Mr. Wu is, it is probably because Mrs. Obama chose his one-shouldered white gown to wear on inaugural night in 2009. Mr. Wu's business savvy helped him capitalize on the attention, making him a go-to designer. He is now so closely associated with the first lady that he is seeking to branch out.

"There has been so much press on him dressing the first lady," said his spokeswoman, Anne Fahey, in a polite refusal to discuss Mrs. Obama's fashion, "and we would like to try to keep the focus on what he is currently doing."

This inauguration heralds another four years of Prabal Gurung, Thakoon, Loree Rodkin jewelry and other fashion brand sightings in Mrs. Obama's public appearances. But the first lady's actual influence may last even longer because her approach has crept muselike into the way designers think.

"She's in my head all the time," says New York designer Sophie Theallet, whose clothes Mrs. Obama has favored. "She represents the women I am dressing."