WASHINGTON — For President Trump, who has made Saudi Arabia the fulcrum of his Middle East policy, the possible murder of a Saudi journalist in Turkey is a looming diplomatic crisis. For Mr. Trump’s son-in-law, Jared Kushner, it is a personal reckoning.

More than anyone in the Trump administration, Mr. Kushner has cultivated Saudi Arabia’s crown prince, Mohammed bin Salman — whose family may have played a role in the disappearance of the journalist, Jamal Khashoggi — elevating the prince into a key ally in the Arab world and the White House’s primary interlocutor to the kingdom.

Mr. Kushner championed Prince Mohammed, 33, when the prince was jockeying to be his father’s heir; had dinner with him in Washington and Riyadh, the Saudi capital; promoted a $110 billion weapons sale to his military; and once even hoped that the future king would put a Saudi stamp of approval on his Israeli-Palestinian peace plan.

While the fate of Mr. Khashoggi, a resident of Virginia and a columnist for The Washington Post, remains unclear, allegations that he was killed on the orders of the royal court have thrown Mr. Kushner’s grand bet on Prince Mohammed into doubt.