WASHINGTON — Over the past two decades, Taiwan has slipped from its position atop the list of flash points in the complex relationship between the United States and China. In meetings between President Obama and President Xi Jinping of China, it has typically come up after half a dozen more pressing issues, like trade, cyberattacks and Beijing’s aggressive moves in the South China Sea.

Now, though, in a single protocol-shattering phone call with the president of Taiwan, President-elect Donald J. Trump has thrust it back on the table. Not since President Richard M. Nixon met with Mao Zedong in 1972 — when the two issued the Shanghai Communiqué clarifying the status of Taiwan — has an American leader so shaken up the diplomatic status quo on the issue.

“Taiwan is about to become a more prominent feature of the overall U.S.-China relationship,” said Jon M. Huntsman, who served as ambassador to China during Mr. Obama’s first term. “As a businessman, Donald Trump is used to looking for leverage in any relationship. A President Trump is likely to see Taiwan as a useful leverage point.”

In the short run, Mr. Trump has rattled the entire region. Representatives of several Asian countries contacted the White House on Saturday to express concern, according to a senior administration official.