WASHINGTON — Nikki Haley, the U.S. ambassador to the United Nations, announced her resignation Tuesday in a move that President Donald Trump said had been in the works for months, but which caught many White House and officials and congressional Republicans by surprise.

Trump, making the announcement at the White House, told reporters that Haley had informed him about six months ago that at the end of a two-year period on the job, she’d want to take a break. The resignation will take effect at the end of the year.

“She’s done a fantastic job, and we’ve done a fantastic job together,” Trump said, adding that he’d be happy to have her back in another position.

Haley’s departure marked one of the rare examples of a senior Trump administration official making a graceful exit. The president heaped praise on her, saying she brought glamour and importance to the position. He allowed her to address reporters from the Oval Office, a departure from the abrupt tweets Trump often uses to announce high-level staff changes.

Trump said he would name a successor within the next two or three weeks.

Haley, in turn, thanked Trump and praised members of his family. She also moved quickly to squelch speculation about her political ambitions.

“No, I am not running in 2020,” she said, adding she would campaign for Trump’s re-election.

Haley, who called herself a “lucky girl,” said she was leaving because she needed to take time out after an intense six years as governor of South Carolina — which included a hurricane, a major flood and mass shootings — directly followed by two years at the United Nations.

Haley also praised the effectiveness of Trump’s foreign policy efforts, which have drawn widespread criticism.

“Now, the United States is respected,” she said. “Countries may not like what we do, but they respect what we do.” A recent survey by the nonpartisan Pew Research Center showed public opinion toward the U.S. has plummeted in many countries since Trump took office.

Haley cited Trump’s tough trade policy, his decision to leave the Iran nuclear deal and the move of the U.S. Embassy in Israel to Jerusalem, which infuriated many allies and regional partners.

Trump, she said, is “showing the rest of the world we will put our embassy where we want to put our embassy.”

Haley, who is not personally wealthy, hinted in her resignation letter to Trump that she is headed to the private sector. Haley has two children to put through college and the potential to make much more money in the business world.

She said her departure matched her belief that those in government should have term limits. She has served at the U.N. since the start of Trump’s presidency.

The decision to announce the latest shake-up came less than a month before the midterm elections, even as the White House has made a concerted effort to hold off on major changes — at the Justice Department and elsewhere — before then.

Trump was asked why the announcement was made now since Haley is staying until the end of the year.

Instead of answering directly, he recounted how she has had to work on tough issues, such as Iran and North Korea.

White House officials had sought to put a hold on record-setting administration turnover in the run-up to the Nov. 6 elections, with aides being asked months ago to step down or commit to stay through Election Day to avoid adding to a sense of turmoil.

Still, the prospect of post-midterm changes has loomed over the West Wing, and Haley’s exit was one of those discussed, according to a senior administration official not authorized to publicly discuss private conversations.

Despite Trump’s calm words, her sudden announcement rattled a number in the White House, who speculated that the timing was meant to preserve the ambassador’s own political future, according to the official and another White House official.

The official noted that their conversation coincided with the appointment of Mike Pompeo as secretary of state and John Bolton as national security adviser. Haley had expressed some frustration that her voice had been diminished as the two men became the aggressive new faces of Trump’s foreign policy, the official said.

More recently, there was an awkward moment at the U.N., when Trump’s boasting of American economic strength under his leadership drew laughter at a General Assembly session. He insisted later that the delegates were laughing with him, not at him.

The six-month timeline also coincides with a high-profile spat between Haley and the White House in April, when she drew the president’s ire for previewing in a television appearance the administration’s planned imposition of a new round of sanctions on Russia. When the sanctions never materialized, White House officials said the plans had changed without Haley being briefed, and top economic adviser Larry Kudlow suggested Haley was confused.

“I don’t get confused,” Haley said in a sharply worded rejoinder to the West Wing.

Haley, 46, was appointed to the U.N. post in November 2016 and last month coordinated Trump’s second trip to the United Nations, including his first time chairing the Security Council.

At the U.N., Haley helped spearhead the administration’s efforts to combat what it alleged to be anti-American and anti-Israel actions by the international body, including the U.S. decision to leave the Human Rights Council and to stop funding the U.N. agency for Palestinian Refugees, known as UNRWA.

Haley also secured three successively tougher Security Council sanction resolutions against North Korea and an arms embargo against South Sudan.

Associated Press contributed.