The American ambassador to China, Terry Branstad, is stepping down, Secretary of State Mike Pompeo announced on Monday, after a tenure that paralleled a sharp deterioration in relations between China and the United States.

       Mr. Branstad, who twice served as governor of Iowa and was a crucial early supporter of Donald J. Trump’s presidential candidacy in 2016, arrived in Beijing in the summer of 2017 with high hopes of using a personal connection to China’s leader, Xi Jinping, to build stronger ties.

       Instead, he found himself on the front lines of President Trump’s trade war and, by this year, a downward spiral of tensions that, to many, has heralded a new era of Cold War-like confrontation between the world’s two largest economies.

       The reasons for Mr. Branstad’s departure now, only months before the presidential election, were not immediately clear. The Embassy in Beijing did not immediately comment on the announcement by Mr. Pompeo, who said on Twitter that the president had chosen Mr. Branstad “because his decades long experience with China made him the best person to represent the Administration and to defend American interests and ideals.”

       Mr. Branstad, who is 73, kept a lower profile than some of his predecessors at the embassy, though that in part reflected Mr. Trump’s outsize role as his own public messenger on China. The ambassador met privately with Mr. Xi, whom he first met while the future Chinese leader was a county official touring rural America, but the personal relationship failed to translate into closer ties.

       Mr. Branstad traveled the country, making a rare trip to Tibet in 2019, but his efforts to build good will often faced resistance from the Chinese as tensions rose over trade and the coronavirus pandemic.

       Last week the State Department complained that the People’s Daily, the main newspaper of the Communist Party, refused to publish an op-ed by Mr. Branstad, who hoped to take his message directly to Chinese readers as his counterpart in Washington, Cui Tiankai, often does.

       “The People’s Daily’s response once again exposes the Chinese Communist Party’s fear of free speech and serious intellectual debate — as well as Beijing’s hypocrisy when it complains about lack of fair and reciprocal treatment in other countries,” the department said in a statement.

       The People’s Daily responded by saying that Mr. Branstad’s article was “full of loopholes and seriously inconsistent with the facts.”