Styrofoam packing peanuts littered an empty office in the Rayburn House Office Building across from the Capitol on Monday morning as two moving men unpacked a plush couch, an upholstered armchair, lamps and a lucite side table.

Representative Elise Stefanik of New York was back.

This had not been the plan.

Ms. Stefanik, the self-proclaimed “ultra MAGA” warrior whom President Trump nominated to serve as ambassador to the United Nations, had expected to sail through her Senate confirmation vote, which was to be scheduled in early April.

So she boxed up her office. She sent off her longtime chief of staff, Patrick Hester, to start a new job at the State Department, where he ended up working for seven days. She completed a “farewell tour” of her district, checked out schools for her son in New York City and was looking forward to moving into the $15 million Manhattan penthouse that comes with what is considered a fairly cushy job.

Instead, Ms. Stefanik was back here on Capitol Hill amid the peanuts, contemplating her next steps and pinning most of the blame for what happened on Speaker Mike Johnson.

To detractors, the president’s decision to pull Ms. Stefanik’s nomination was something akin to karmic comeuppance for a Republican lawmaker who was elected as a moderate but tacked unapologetically to the MAGA right, coming to personify the opportunistic shape-shifting that has gripped her party in the age of Mr. Trump.

Ms. Stefanik’s plight seemed to crystallize in one succinct cautionary tale the limits of loyalty in the MAGA universe. Even one of the president’s most stalwart defenders, an effective ally since his first impeachment trial, ultimately did not get what she had long been promised.