International diplomacy is a world of careful rituals, hierarchy and credentials. But when the Israeli ambassador to the United States, Ron Dermer, wanted to communicate with Donald J. Trump, he ended up on two occasions in the Manhattan office of a young man with no government experience, no political background and no official title in the Trump campaign: Jared Kushner.

Mr. Kushner held court at length with Mr. Dermer, doing his best to engage in the same sort of high-level conversation that the ambassador conducted with career diplomats and policy experts from Hillary Clinton’s campaign.

A 35-year-old real estate developer, investor and newspaper publisher, Mr. Kushner derives his authority in the campaign not from a traditional résumé but from a marital vow. He is Mr. Trump’s son-in-law.

Yet in a gradual but unmistakable fashion, Mr. Kushner has become involved in virtually every facet of the Trump presidential operation, so much so that many inside and out of it increasingly see him as a de facto campaign manager. Mr. Kushner, who is married to Mr. Trump’s daughter Ivanka, helped recruit a sorely needed director of communications, oversaw the creation of an online fund-raising system and has had a hand in drafting Mr. Trump’s few policy speeches. And now that Mr. Trump has secured the Republican nomination, Mr. Kushner is counseling his father-in-law on the selection of a running mate.