A protester sleeps on the streets outside the Hong Kong Government Complex at sunrise. (Paula Bronstein/Getty Images)

IT’S HARD not to be inspired by the images of crowds in the center of Hong Kong peacefully demonstrating in favor of democracy, their unlikely symbol not a clenched fist but an open umbrella. But it’s also difficult not to remember the similar mass demonstrations that filled Beijing’s Tiananmen Square 25 years ago and how those ended. The pessimistic consensus in and outside China is that the Communist party leadership of Xi Jinping, which has adopted a hard line against political dissent, is likely to forcibly crush this protest movement if it persists, just as the last one was crushed.

Beijing, however, has not acted yet; police in Hong Kong backed off on Monday and Tuesday after their use of tear gas over the weekend brought more people to the streets. Chinese authorities probably are weighing the risks of allowing the street occupations to continue against those of initiating a crackdown. That makes this a crucial moment for the United States to send a clear message to Mr. Xi: that repression is unacceptable and will damage China’s relations with the democratic world.

Unfortunately, the Obama administration’s response so far has been gallingly timid. White House and State Department spokesmen have carefully avoided offering explicit support for the demonstrators’ demands for free elections for the city’s leader, rather than a managed choice among nominees approved by Beijing. They have urged the demonstrators to be peaceful, though only the police have resorted to violence.

As a supporter of the 1984 agreement under which Hong Kong was transferred from British to Chinese rule, the United States has an obligation to speak up when China violates the spirit of its promise to allow an elected government — as it clearly has. Yet the U.S. Consulate in Hong Kong went so far as to declare that “we do not take sides in the discussion of Hong Kong’s political development, nor do we support any particular individuals or groups involved in it.”

Even more concerning is U.S. nonchalance about a possible crackdown. Asked about speculation that Chinese military units stationed in Hong Kong could be used against the protesters, the State Department’s spokesman said Monday that “I have not seen that potential at this point in time. I can check with our team to see if that’s a concern we have.”

State would do well to check with Chinese dissidents such as Yang Jianli, Teng Biao and Hu Jia, who know the regime’s capacity for repression all too well. In an oped published by the Wall Street Journal on Tuesday, the three men pointed out that Chinese officials “have threatened repeatedly that Hong Kong-based units of China’s People’s Liberation Army will use force to suppress peaceful demonstrations,” adding, “this tragic outcome is becoming more likely.”

After the 1989 Tiananmen massacre, President George H.W. Bush and Congress imposed tough sanctions on China, though Mr. Bush soon backed down. Since then China has grown into a major power that is more resistant to outside pressure. The United States cannot protect Hong Kong’s democracy movement if Mr. Xi decides to crush it. But it can and should support its demand for genuine democracy and let China know that the use of force would have consequences for U.S.-Chinese relations.