When Heather Nauert, the State Department spokeswoman and former “Fox & Friends” host, withdrew from consideration for United Nations ambassador, President Trump averted a potentially difficult Senate confirmation struggle over what some critics have called her insufficient experience. But Ms. Nauert’s withdrawal also underscored his challenges in filling the job.

A terse announcement on Saturday night that Ms. Nauert no longer wished to be considered because of family considerations appeared to put Mr. Trump back where he started when his first ambassador, Nikki R. Haley, announced last October that she was leaving at the end of 2018.

Ms. Haley’s departure, with no word on a successor, has left the Trump administration devoid of a high-profile presence at the United Nations, the world’s most prominent diplomatic stage, for nearly two months. Here is a look at the role of the ambassador and whether a prolonged vacancy poses problems for the United States:

A U.N. ambassador is in many ways America’s face to the world.

Over the 73 years since the United Nations was founded, successive White House administrations have attached enormous weight to the Americans chosen to represent the host nation. They have included former governors and future presidents (Adlai Stevenson II, George H.W. Bush), prominent jurists (Arthur Goldberg) and a range of politicians and experienced diplomats from both major political parties (Jeane Kirkpatrick, Andrew Young, Madeleine Albright, Richard Holbrooke, John Negroponte).

“The United Nations ambassador is a premier diplomatic role, one of the top foreign policy jobs that anyone can have,” said Victoria K. Holt, a former State Department diplomat and fellow at the Henry L. Stimson Center, a Washington-based policy research center. The role is especially important at the United Nations, Ms. Holt said, “a place of persuasion and relationships.”