BOSTON, July 28 - Alexandra Kerry had just finished autographing a young man's campaign T-shirt - "I told him on eBay it'll be worth 10 cents" - after a rally in Sioux City, Iowa, when someone summoned her behind a tent to meet Kenneth Cole.

The Kenneth Cole, he of shoes and bags, and, more important to an aspiring filmmaker like Ms. Kerry, a member of the board of the Sundance Institute.

He asked who made her jeans. She had no idea, arching her back to read the label.

Ms. Kerry ambled over to the television reporters waiting for interviews, only to be stopped by a Secret Service agent because she was not wearing the proper badge.

"It is a bit like being a rock star and not really having an instrument to play," Ms. Kerry, 30, said of her new life as the daughter of the man who would be president. "You're constantly communicating and constantly emoting. I want to be careful about the word performance, because it's very genuine. But you're constantly on stage."

On Thursday night, Ms. Kerry and her sister Vanessa, 27, will literally be on stage here at the Democratic National Convention to help introduce Senator John Kerry to the thousands of delegates nominating him for president and a national television audience. Their short speaking stints are a sign not only of the growing presence of offspring in the Kerry-Edwards image-making operation, but also of the newly prominent roles that politicians' children have generally as the ultimate proof of their parents' values.

This week, the Kerry daughters have been stars on the party circuit, celebrated on Wednesday with Champagne and smoothie shooters at a brunch sponsored by their predecessors, the daughters of Al Gore, that drew Ben Affleck, the actor, and André 3000 of the rap group Outkast.

They have been featured on umpteen television talk shows, met countless state delegations and even held a news conference on Wednesday.

As proved by the reaction of the men in the back of the room at the brunch - "Vanessa Kerry's so hot," one said - the spotlight is often more about show than substance, something that makes these ambitious young women uncomfortable.

They bristle at the "babe battle'' shaping up between the Kerry children and the Bush twins, who were featured in strapless couture in Vogue this month. Alexandra and Vanessa Kerry will appear in the September issue.

Having spent childhood largely shielded from their father's long public career, the sisters stepped out slowly, with Vanessa Kerry taking time off medical school last fall to barnstorm students' dormitories in Iowa and New Hampshire. Alexandra appeared only at big moments like election nights before finishing film school this spring. Now, they are often frustrated by false rumors about whom they are dating and excess attention to what they wear, struggling to maintain a modicum of privacy while simultaneously trying to gain maximum attention for the cause.

"By accident of birth, you suddenly find yourself in a place where you can communicate messages about issues that matter to you enormously," said Rebecca Lieberman, who campaigned for her father in 2000, when he was the Democrats' vice-presidential candidate, and this year.

The role fluctuates between substantive surrogate and "being an accessory," Ms. Lieberman added.

"And you need to adjust to thinking of yourself that way,'' she said. " 'I'm here in the shot I'm just like a tie or something.' "

The Kerrys are part of a Brady Bunch brood that includes their stepbrothers, Chris Heinz (one of People magazine's "50 hottest bachelors'') and Andre Heinz (known for his impression of Arnold Schwarzenegger) and the adorable Edwardses, Jack, 4; Emma Claire, 6; and Cate, 22.

"These girls are great, most of the time," Cate Edwards said at the brunch." They've become sort of my sisters."

On the Republican side, Barbara and Jenna Bush, also 22, also made debuts on the campaign trail, with Jenna Bush immediately drawing attention by sticking out her tongue at reporters. Noting that "a voter in, say, 1848, wouldn't have dreamt of trying to figure out how much Zachary Taylor's children loved him," Michael Beschloss, the presidential historian, said the additional exposure was part of a broader evolution in how voters chose candidates, now based on comfort with character as much as policies.

"You're trying to find a window into the guy's soul,'' Mr. Beschloss said. "You draw conclusions from what someone's friends and family are like. It doesn't give you too much insight into a candidate's tax policy. But it does illuminate what kind of person he is."

When Mr. Kerry's children appear at fund-raisers and forums, they are just as likely to spin stories of a sweet man who sent "care packages'' of chocolate chip cookies while they were in college as to give talking points on health care or the environment.

If some people find Mr. Kerry stiff, "A" and "Ness," as the family calls them, say he is goofy. On the road, they play tricks on him like sneaking into his hotel room and spreading on his bed a buffet of all the junk food people had given them that day.

In their first joint appearance, filling in on the spur of the moment when Mr. Kerry was delayed one day in Iowa, "we just did our sibling banter, but with two microphones," Vanessa Kerry said.

With a pen stuck in her blond ponytail and gold hoop earrings the size of bracelets, Ms. Kerry said one hard lesson had been that her dry sarcastic sense of humor did not translate well in print. That joke about wanting her Secret Service nickname to be "the hot one" did nothing to dispel discussion about her appearance.

She uses fake last names on her hospital ID's at the Harvard Medical School and, when she was moving a few weeks ago, did not set straight a passer-by who asked whether anyone had every told her she looked a lot like Senator Kerry's daughter.

"People want to make us characters," Ms. Kerry said in an interview. "I can't stand the smile-wave thing."

Acknowledging that "this sounds contrived," she said she decided last fall to join the campaign, because "pretty much everything I care about is going to be impacted in this election." She cited global health problems like AIDS.

"I just realized I had an incredibly unique opportunity to affect my career," said Ms. Kerry, who will spend a week next month working on infectious diseases in a developing country, and return to campaign in the fall from a Fulbright fellowship in London. "You see your father who you love and believe in, and you want to be a part of it."

Alexandra Kerry is, perhaps, even more reluctant, preferring to pull out her camera - she has so far shot 40 hours for an insider's campaign documentary - than be part of the picture. An unfortunate confluence of flashbulbs sent startlingly see-through pictures of her black dress at the Cannes film festival careening around the Internet, and she would rather give her dad notes on his speeches or talk to his message men about staging than speak out herself.

Trotted out for a conference call about Mr. Kerry's preconvention tour, Alexandra Kerry referred queries about why her father was born in Colorado and how long he lived there to a campaign aide, saying, "This is going to be the embarrassing responsibility of the daughter who doesn't know the details."

The women said they learned more about Mr. Kerry's Vietnam War experience from the campaign than they did growing up, reading his letters home for the first time in "Tour of Duty,'' Douglas Brinkley's biography.

"The only thing I ever remember him lying about was how he got a Silver Star," Vanessa Kerry said, recalling that she asked about it was she was 7. "He didn't tell me he killed a man."

Though Alexandra Kerry is older, Vanessa Kerry often acts more mature, warning her sister not to talk long on a cellphone call to their mother in Italy, buying her salad and Spree candy - "This is the 'wonderall' for people who feel ill," she said - and paying the extra $25 for her sister's overweight suitcase at the airport. Vanessa Kerry, who has a voracious metabolism, never travels without a can of almonds and gum. Her sister is more apt to leave something behind - her hanging bag, her camera or her shampoo.

They giggle like girlfriends, the campaign bringing them closer, because few share the bizarre experience of hearing father's familiar voice booming from the television sets as they walk through airports.

Riding in a van last week from the airport to a hotel in Denver, Vanessa Kerry thrust a copy of the latest Esquire magazine at her sister's lap.

"Dad apparently has the world's largest head," she cackled, pointing to Mr. Kerry's face as it spilled over two pages. "That's actual size!"

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