FILE - In this Nov. 28, 2013 file photo, Joseph E. Macmanus, permanent U.S. Representative to the U.N. office in Vienna, gives a news conference after a meeting by the International Atomic Energy Agency (IAEA) board of governors in Vienna, Austria. A Republican senator has been blocking President Donald Trump’s choice, Macmanus, to become the next ambassador to Colombia, which could force Trump to either select a new person or wait until a new Congress begins in January 2019 to nominate Macmanus again. (AP Photo/Ronald Zak, File)

FILE - In this Nov. 28, 2013 file photo, Joseph E. Macmanus, permanent U.S. Representative to the U.N. office in Vienna, gives a news conference after a meeting by the International Atomic Energy Agency (IAEA) board of governors in Vienna, Austria. A Republican senator has been blocking President Donald Trump’s choice, Macmanus, to become the next ambassador to Colombia, which could force Trump to either select a new person or wait until a new Congress begins in January 2019 to nominate Macmanus again. (AP Photo/Ronald Zak, File)

BOGOTA, Colombia (AP) — A Republican senator has been blocking President Donald Trump’s choice to become the next ambassador to Colombia, citing concerns that one of the State Department’s most senior career diplomats mishandled the aftermath of the deadly assault on the U.S. consulate in Benghazi, Libya.

Sen. Mike Lee, R-Utah, led a small chorus of conservatives who raised doubts about Joseph Macmanus’ nomination almost as soon as it was announced last November and has used a procedural rule known as a “hold” to prevent any vote from taking place.

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If he doesn’t lift it, Trump could be forced to either select a new person or wait until a new Congress begins in January to nominate Macmanus again.

A spokesperson for Lee’s office said the senator has no plans to remove the hold but declined further comment.

Macmanus is the longest delayed of 20 ambassadorial nominees awaiting confirmation, according to the American Foreign Service Association. Another 41 top ambassadorial posts are vacant.

The fact that Macmanus has been unable to take up an assignment traditionally reserved for a career diplomat at the same time other strategic ambassadorial postings including Mexico and Australia remain unfilled “is a sign of Washington’s dysfunction,” said Adam Isacson, a senior analyst at the Washington Office on Latin America.

Macmanus was chosen to replace Ambassador Kevin Whitaker at a crucial time in relations with Washington’s staunchest ally in the region.

Colombia offered to deploy troops to help the U.S.-led mission in Afghanistan and is one of the biggest recipients of American foreign aid outside the Middle East. But the Trump administration last year threatened to decertify Colombia as a partner in the war on drugs because of over a record boom in cocaine production — a step that could affect anti-narcotics cooperation between the two countries. The White House last week said land dedicated to the production of coca surged 11 percent in 2017 to 209,000 hectares (800 square miles).

President-elect Ivan Duque, who met last week in Washington with Secretary of State Mike Pompeo, has vowed to turn the numbers around and said he’s even considering bringing back a controversial, U.S.-backed aerial fumigation program that was scrapped in 2015 over health concerns.

Several Republicans expressed reservations about Macmanus when he was nominated. Lee, in December, called him “the wrong man for Colombia,” saying he doesn’t share Trump’s “America First” foreign policy and voicing concerns about his perceived closeness to former Secretary of State Clinton.

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Macmanus was Clinton’s executive secretary and helped handle the aftermath of the terror attack on the U.S. consulate in Benghazi that killed Ambassador Chris Stevens and three other Americans. Republicans accused then-President Barack Obama and Clinton of intentionally misleading the public and stonewalling congressional investigators. A final report by a Republican-led congressional panel found no wrongdoing by Clinton, but faulted the Obama administration for security deficiencies at the Libyan outpost and a slow response to the attacks.

During his confirmation hearing in March, Macmanus said that he never intentionally misled the public.

While Macmanus’ role in the Benghazi situation has made him a lightning rod for conservative criticism, he also served in the same job under Republican Secretaries of State Condoleezza Rice and Rex Tillerson. Between 2012 and 2014, he was the U.S. representative to the International Atomic Energy Agency, and in 2015 served as the interim coordinator for the Iran nuclear deal — another position called into question by Lee.

“What is Macmanus committed to? It is hard to say for certain, but his background is cause for concern,” Lee, who is not a member of the Foreign Relations Committee, said in December.

Even some Republican supporters of Macmanus have expressed concern at his lack of recent experience in anti-narcotics policy or Latin America, though he did serve in Mexico and El Salvador early in his career.

“While Macmanus’ career as a U.S. foreign service officer is commendable, his lack of relevant and recent involvement in Western Hemisphere affairs will significantly undermine his effectiveness,” the conservative Heritage Foundation said in a report last year. “Macmanus has been absent from the region far too long. And the problems pressing on Colombia are too urgent to accommodate a learning curve.”

Some of Macmanus’ backers were hopeful Lee’s objections had eased after Pompeo didn’t put forward another candidate upon being sworn in as secretary of state, and the Senate Foreign Relations Committee in May backed his nomination, sending it to the Senate floor for a vote.

But Lee’s hold has so far prevented a vote from ever taking place.

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Associated Press writer Matthew Lee in Washington contributed to this report.

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Joshua Goodman on Twitter: https://twitter.com/APjoshgoodman