But there is so much more to a first lady's fashion identity than that one Cinderella moment. Mary Todd Lincoln was a shopaholic who took a private rail car to New York for retail therapy. Grace Coolidge wore drop-waist flapper dresses that were short -- but not too short -- making herself a model for women looking to Charleston through the Roaring '20s without being too provocative. Eleanor Roosevelt was the first first lady to be snapped in a bathing suit; Lady Bird Johnson sponsored the first White House fashion show to draw attention to the environment and promote travel in America.
All of which is to acknowledge that first lady fashion-watching is a national pastime. What they've worn on vacation, foreign visits and at bedtime has been chronicled in newspapers and magazines and, later, on TV. They've started hairstyle trends from chignons to bouffants to bangs. They've even had colors named after them: Eleanor blue, Mamie pink and Nancy red.
At times, fashion has been used for political purposes. "When the cotton industry was spiraling, Lou Hoover [first lady from 1929 to 1933] was asked to pose for photographs wearing cotton dresses," says first lady historian Carl Sferrazza Anthony. (Hoover tried to popularize cotton for evening clothes, but it didn't take off.)
And although there's no written rule that first ladies must wear American-made clothing, the tradition started with Martha Washington (1789-97), who was encouraged to wear "homespun" clothing instead of British fashions in the aftermath of the Revolutionary War.
Even before the role became a public one in the late 19th century, first ladies wielded enormous symbolic power. And today, they still reflect and refract the image the American public has of itself. Many have used style to set a tone for their husband's presidency, starting with that first first lady. Washington "knew instinctively how clothing conveyed a message. And the image she projected was carefully calculated to be simultaneously republican and elegant. She dressed plainly but in the finest fabrics," according to Helen Bryan's 2002 book, "Martha Washington: First Lady of Liberty."
Dolley Madison elevated the office by dressing smartly in Empire-waist dresses with an ample dose of cleavage, and turbans adorned with bird of paradise feathers, first as the official White House hostess for Thomas Jefferson and later as James Madison's first lady (1809-17). She was so beloved that she became the standard by which all other first ladies were long compared.
"Even though people had fought a revolution against monarchy, the only vocabulary they had was monarchy," says Catherine Allgor, visiting professor of history at Claremont McKenna College and author of "A Perfect Union: Dolley Madison and the Creation of the American Nation."
"Dolley Madison understood the populace needed reassurance that the nation was going to hold. She was Queen Dolley because there was a craving for the imprint of legitimacy that monarchy gave. And one of the jobs of first lady is to send out psychological and emotional messages to the country."
Lincoln (1861-65) wanted the same magic. But she went on shopping sprees and accepted unlimited credit at department stores while men were dying on the battlefield during the Civil War. This was reported in the press at the time and caused her husband great embarrassment.
In 1864, when the president was up for reelection, Mary Todd Lincoln told a friend she had spent $27,000 on clothing, according to the 1998 book "America's First Ladies: Changing Expectations," by Barbara Silberdick Feinberg.
Abraham Lincoln's annual salary was $25,000.
After he died, the first lady was in so much debt that she was forced to sell clothes on consignment and in 1867 to host "Mrs. Lincoln's Second-Hand Clothing Sale." It was a flop.
Eleanor Roosevelt (1933-45) had a cozy arrangement of her own with New York retailer Arnold Constable -- during the Great Depression no less. "She would pose in the clothes for pictures that the store would distribute to the press," says Maurine Beasley, a professor at the Philip Merrill College of Journalism at the University of Maryland and author of "First Ladies and the Press." It was an advertising tool for the store, in exchange for which Roosevelt would receive a discount.
Fifty years later, Nancy Reagan (1981-89) would be attacked by some for wearing designer freebies from Adolfo, Bill Blass and James Galanos when the economy was on unsure footing.
Other first ladies proved to be an inspiration for advertising and commerce. After marrying in the White House in 1886, the 21-year-old Frances Cleveland (1886-89, 1893-97) became an instant celebrity. The use of color in advertising was new, and companies used her name and youthful face without permission to hawk goods including bitters and underwear. Her hairstyle -- a low chignon -- was much imitated. And when a gossip report falsely claimed she had abandoned the bustle, she went ahead and did it, and so did the rest of America.
"Every now and then a first lady seems like a breath of fresh air. And she was one of those people," explains Lisa Kathleen Graddy, curator of the "First Ladies: Political Role and Public Image" exhibit at the Smithsonian.
It wasn't the first time the commercial industry had capitalized on the first lady role, and it wouldn't be the last. Barbara Bush (1989-93) had a line of Kenneth Jay Lane jewelry named after her, playing off her trademark triple strand of faux pearls, which is similar to Obama's. ("Pearls are virtuous jewelry, quintessentially American," Allgor notes. "Diamonds are too aristocratic.")
Coolidge (1923 to 1929), following the unpopular Florence Harding, was one of a handful of first ladies to attain fashion icon status. In tailored suits, drop-waist tunics and skirts, Coolidge "upheld the image of the wholesome flapper," Beasley says. At night, she dressed like a movie star in gold lamé and lace and, thanks to movie reels, she cut a familiar form for the American public. A baseball enthusiast, she was also an early adopter of modern sportswear who liked to hike, play tennis and ride.
Jacqueline Kennedy (1961-63) proved that Europe did not have a monopoly on good taste. She may have hailed from the upper crust, but she had popular appeal. Her uncluttered elegance, influenced by Hubert de Givenchy's French chic and executed by American designer Oleg Cassini during the White House years, was a boon to the fashion industry, as her trademark evening sheaths, pillbox hats and Capri pants were copied and sold. In March 1961, Women's Wear Daily wrote that the Jackie look had become "part of the retail ad language." Photographers followed her every move, even on the beach in Ravello, Italy. She was young, vigorous and attractive.
Hillary Rodham Clinton's (1993-2001) fashion identity was sometimes confused, as she struggled between dressing as a mother figure and a working woman. There were a lot of schoolgirl headbands, straw hats and unfortunate prints before she eventually settled on her signature pantsuits. And her changing hairstyles were critiqued almost as often as her policies. Clinton poked fun at herself last year when she named her worst fashion moments for Us Weekly magazine.
The Bush first ladies' style legacies are minimal. In fact, it's difficult to recall anything specific from either of their wardrobes. So it's no wonder there's excitement over what Michelle Obama will be wearing as first lady. "She has a lot of leeway because she is like us," Allgor says. Instead of buying copies of what she wears, we can buy the real thing from J. Crew and Banana Republic.
"And when they dress her for fabulous events, there's no uneasiness, because we know that she wouldn't be wearing those things in real life," Allgor says. "It's the closest thing to what would happen if you or me became first lady. It's like 'Princess Diaries.' "
At least for now, we're under her spell.