Jeff Flake, a former senator and one of the most outspoken Republican foes of Donald Trump, was nominated Tuesday by President Joe Biden as ambassador to Turkey, tasked with managing one of the most complicated US alliances.

Flake, who unlike many Republicans has never retracted his criticism of Trump and went so far as to campaign for Biden in last year's election, said he was "humbled and honored" to be chosen as ambassador.

"This is a pivotal post at an important time for both of our countries," Flake wrote in a blog post as his nomination was announced in a routine White House statement.

"With this nomination, the Biden administration reaffirms the best tradition of American foreign policy and diplomacy: the credo that partisan politics should stop at the water's edge."

Turkey, which has experienced major tensions with Washington in recent years but is also a vital partner in key areas such as Afghanistan, was one of the few prominent posts where Biden had left in place a Trump appointee -- David Satterfield, a career diplomat with vast experience in the Middle East.

Flake's confirmation in the Senate appears virtually assured as Democrats are in control and he remains friendly with many Republican colleagues. Republican Senator Mike Lee of Utah said that Biden showed "wisdom" in choosing him.

Vociferous Trump critic

Flake, 58, has long been expected to be tapped for a diplomatic post but speculation had focused on an ambassadorship in Africa. Flake served as a Mormon missionary in South Africa and Zimbabwe and was active on African issues as a senator.

Biden earlier nominated another Republican critic of Trump from Arizona -- Cindy McCain, widow of veteran senator John McCain -- to be the US ambassador to the UN Food and Agriculture Organization in Rome.

With a boyish smile and an upright moral code commonly associated with the Church of Jesus Christ of Latter-Day Saints, Flake clashed with Trump from the beginning of the brash tycoon's political ascent -– even though Flake largely supported Trump's legislative agenda.

In a speech before Harvard Law School in 2018, Flake said that "our presidency has been debased by a figure who has a seemingly bottomless appetite for destruction and division and only a passing familiarity with how the Constitution works."

By then, Flake had already decided not to seek a second term in the Senate as an infuriated Trump purged the party.

Near the end of his term, Flake made headlines by appearing visibly distraught after two women approached him in an elevator to say how they survived rape and urge him to block Supreme Court nominee Brett Kavanaugh -- leading Flake to hold up the nomination temporarily until the FBI probed allegations against Kavanaugh.

Complex relationship

Flake will have a delicate task in handling Turkish President Recep Tayyip Erdogan –- who developed such a close relationship with Trump that he reportedly would call the US leader directly and without aides when he was golfing.

Biden as a candidate branded Erdogan, who has roots in political Islam and has launched a sweeping crackdown after a failed coup attempt in 2016, an "autocrat" and vowed to support the opposition.

In April, Biden became the first US president to recognize the mass killings of Armenians by the Ottoman Empire as genocide, a categorization from which US leaders had shied away for decades under pressure from Turkey.

Relations have also been strained for years by Turkey's purchase of an advanced air defense system from Russia, which defied US warnings that it was jeopardizing its role in the NATO alliance.

But Biden and Erdogan appeared to find ways of cooperation when they met in June in Brussels on the sidelines of a NATO summit.

Erdogan offered Turkish help in securing Kabul's international airport, seen as crucial for the country's long-term stability after Biden ordered a pullout of remaining US troops and as Taliban insurgents make rapid gains.