When Michelle Obama takes the stage at the Democratic National Convention in Charlotte on Tuesday night, she will once again embrace a role that wives of presidential contenders have now played for two decades: humanizer of their husbands, dogged advocate for their men. Historians and pollsters say there is no data to show whether these speeches make a difference come November, but they have become essential rituals since Barbara Bush addressed the Republican convention in Houston in 1992.

In the beginning, however, political wives did not give political speeches.

When Martha Washington was on her way to New York in 1789 to join her husband, George, fresh off his inauguration, she “decided to emerge [from her coach] and spontaneously address the people, thanking them . . . for so warmly welcoming her with parades, fireworks, cheers, etc,” Carl Sferrazza Anthony, an author of several books about first ladies and the consulting historian at the National First Ladies’ Library, writes in an e-mail.

But there were no reporters on hand to transcribe her remarks, and there was none of the preparation, scrutiny and fanfare that have come to accompany speeches made by first ladies and those who aspire to that role.

It was Barbara Bush who set the blueprint for the modern first lady convention speech with her address in 1992.

“As convention speeches go, her 1992 speech is an interesting one. It tried to be very inclusive. The intent was to show that everyone was important to [George] Bush,” says Myra Gutin, a professor of communication at Rider University and a first lady scholar.

With self-deprecating humor, Barbara Bush said, “There’s something not quite right here . . . speeches by President Ronald Reagan, President Gerald Ford, Secretary Jack Kemp, Senator Phil Graham and Barbara Bush?”

Anthony explains that Barbara Bush’s role was to appeal to moderate voters after Pat Buchanan’s open opposition to gay rights and abortion earlier in the week. With that, she set the precedent for what has followed — highly scripted, targeted speeches designed to showcase a softer side of the potential president and appeal to the niche group that isn’t really a niche group: women.

Women weren’t allowed to attend party conventions when they began in 1832, and presidential nominees usually didn’t attend them either, at least not until after World War II, according to Anthony. A presidential nominee learned of his party’s nomination on Notification Day, when a group of delegate officials and members of the media would show up at the nominee’s home. Wives would sometimes peer out of first-floor windows (Mary Lincoln in 1860) or eavesdrop behind doors (Edith Roosevelt in 1904).

It wasn’t until 1940, when Eleanor Roosevelt addressed delegates at the Democratic National Convention in Chicago, that an incumbent first lady spoke to party delegates officially. Franklin D. Roosevelt was seeking an unprecedented third term and asked Eleanor to mollify delegates miffed at his choice of Henry Wallace as vice presidential nominee.

“You will have to rise above considerations which are narrow and partisan,” Eleanor chided, according to a transcript of the speech in George Washington University’s Eleanor Roosevelt Papers Project.

In 1996, two politically ambitious women, Elizabeth Dole and then-first lady Hillary Clinton, went to bat for their husbands, prompting Pulitzer Prize-winning columnist Ellen Goodman to dub them Hillabeth and Elizary.

Dole was up first at the Republicans’ gathering. She circulated the convention floor like a talk-show host, switching mikes when an audio problem arose and introducing people in the audience

Two weeks later, Hillary Clinton spoke at the Democratic convention, never leaving the podium.

“In those speeches you hear a role that walks in between the traditional spouse role and the role of a person who might speak in her own right. Each one does explain and justify some of the policy agenda of the husband, [but] doesn’t do it as a spokesperson for him,” says Kathleen Hall Jamieson, a professor of communication and the director of the Annenberg Public Policy Center at the University of Pennsylvania. Both Dole and Clinton went on to successfully run for the Senate.

Teresa Heinz Kerry’s 2004 address to Democratic delegates in Boston went a step further, breaking what Jamieson says is the golden rule of aspiring first lady speeches: It’s not about you.

Kerry didn’t get to any of her husband’s platform points until the 12th minute of her speech.

“It seemed to be advancing a case for her instead of him,” Jamieson says.

That wasn’t an issue for Ann Romney at RNC. She was widely commended for painting a broader picture of a man sometimes considered socially awkward. Nor was it an issue for Michelle Obama in 2008, when she used her own South Side of Chicago background to assuage fears that Barack Obama was too foreign.

Both women made a show of the conjugal symbiosis that has become a feature of modern presidential campaigns, according to Jamieson: “We don’t elect an individual, we elect a couple.”