By conveying the belief that China is straying from its decades-old commitment to preserve Hong Kong’s separate political system, the top American officials are buttressing their argument that a trade treaty with China must have strict enforcement provisions.

They are also sending the message that a violent crackdown in Hong Kong along the lines of the 1989 Tiananmen Square massacre would make China a pariah in the eyes of many Americans and other citizens around the world, rendering it all but impossible to resume trade discussions amid the global backlash.

“Showing support for a peaceful resolution of the situation in Hong Kong could potentially give the U.S. a stronger hand in trade negotiations, under the assumptions that Beijing wants a trade deal badly enough and wants to maintain Hong Kong’s special status in its trade with the U.S.,” said Eswar Prasad, the former head of the International Monetary Fund’s China division. “Both are reasonable assumptions but might be trumped by Beijing’s unwillingness to countenance further protests in Hong Kong that directly challenge Beijing’s authority.”

The Trump administration has already imposed tariffs on $250 billion of Chinese imports and is planning to tax another $300 billion worth. American and Chinese officials have spoken by phone in recent weeks, but no date has been set for a Chinese delegation to visit Washington in September, as had been previously announced. And Mr. Trump has made clear he does not mind keeping the trade war going.

“Unless they’re going to make the right kind of a deal, I’m not ready to make a deal,” he told reporters this week.

Foreign policy officials have grappled in recent weeks with how to get Mr. Trump to take a more forceful stand on Hong Kong. Officials are wary of potentially harsh measures Mr. Xi might take against the protesters. They have become more concerned as videos have emerged of Chinese troops massing across the border in Shenzhen.

Several top aides had urged Mr. Trump to make statements warning the Chinese leadership against using violence. Among them is John R. Bolton, the national security adviser, who views China as a formidable rival. Mr. Trump, on the other hand, said in June that he saw the United States and China as “strategic partners.”