Among the Trump children, she is the acknowledged favorite: “Daddy’s little girl,” as her older brother Donald Jr. once described her. When Ms. Trump and her father are not together, she said, they speak as often as five times a day.

In the Trump family business, he has given her a level of authority none of his wives, or for that matter executives, have ever had. She handles some of the Trump Organization’s biggest deals, including its acquisition of the Doral Resort in Miami, and its agreement to convert the Old Post Office building in Washington, D.C., into a luxury hotel.

And with Mr. Trump’s wife, Melania, the Slovenian former model, less than comfortable on stage, Ivanka Trump has served as a surrogate political spouse for a candidate who may need one more than anyone else in the race.

Cutting a starkly different image from Mr. Trump’s more combative and impulsive public supporters, like Chris Christie or Sarah Palin, not to mention the candidate himself, Ms. Trump, 34, radiates disciplined poise and practiced reserve. While her father uses Twitter as a grenade launcher, she treats her well-tended social media feeds, which are notably politics-free, as marketing tools for the Trump Organization and her own line of women’s clothing and accessories.