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When Hillary Clinton was secretary of state, she relied on Henry A. Kissinger’s counsel. He would send her “astute observations about foreign leaders” and “written reports on his travels.” She would joke with him that smartphones would have made his covert Cold War trip to Beijing impossible.

The two diplomats had a cordial, warm and respectful relationship, based on writings about their interactions during Mrs. Clinton’s tenure at the State Department.

“Kissinger is a friend, and I relied on his counsel when I served as secretary of state,” Mrs. Clinton wrote in The Washington Post, in a positive review of his book “World Order.”

The friendship came back to haunt her in the Democratic presidential debate on Thursday night, when Senator Bernie Sanders pointedly questioned Mrs. Clinton’s foreign policy judgment, saying President Richard M. Nixon’s secretary of state had enabled genocide in Cambodia under Pol Pot.

“I’m proud to say Henry Kissinger is not my friend,” Mr. Sanders said.

For Mrs. Clinton, Mr. Kissinger, now 92, was part of “a fascinating club” of former secretaries of state she turned to when she arrived at Foggy Bottom, writing in her 2014 memoir “Hard Choices” that these diplomats “transcend partisan differences.”

She specifically praised Mr. Kissinger’s diplomatic achievements in China, where he helped Mr. Nixon normalize diplomatic relations with the Communist leadership in a visit to Beijing in 1972. “I was riveted and proud of what America accomplished during what President Nixon called ‘the week that changed the world,’” she wrote.

Indeed, Mr. Kissinger’s trip looms over much of Mrs. Clinton’s experiences in China as President Obama’s top diplomat. Mrs. Clinton sought Mr. Kissinger’s advice when she met with the Chinese state councilor, Dai Bingguo, and the foreign minister, Yang Jiechi, who she said “would become my primary counterparts in the Chinese government.”

“Henry Kissinger had told me how highly he valued his relationship with Dai, whom he found to be one of the most fascinating and open-minded Chinese officials he had ever encountered,” Mrs. Clinton wrote.

The squabbling over Mr. Kissinger echoes the deep divisions over the Vietnam War during the early 1970s, when Mr. Kissinger was a main target of the left for the Nixon administration’s conduct of the war.

She also called on Mr. Kissinger’s advice in other areas, including asking for his help in Mr. Obama’s persuading of Prime Minister Benjamin Netanyahu of Israel to apologize in 2013 to President Recep Tayyip Erdogan of Turkey for the deaths of Turkish citizens in a Gaza flotilla three years earlier.

“I even enlisted Henry Kissinger to make the strategic case to him in August 2011,” she wrote.