IT’S NOT often that Congress is lobbied by tens of thousands of marchers in a foreign city who wave American flags and sing “The Star-Spangled Banner” while demanding action on a specific piece of legislation. But that’s pretty much what happened last weekend in Hong Kong, where a mass pro-democracy movement, after 13 consecutive weeks of demonstrations, has grown savvy about the challenge it faces in seeking concessions from the Communist regime in Beijing.

Hong Kong’s chief executive, Carrie Lam, has belatedly withdrawn the extradition legislation that prompted the initial protests. But her boss, Chinese President Xi Jinping, reportedly has taken a hard line against conceding to the protesters’ more substantive demands, including free elections for the territory’s government — something that Beijing promised when it took over the former British colony in 1997. Instead, the regime is accusing the mostly peaceful demonstrators of employing “terrorism” and has threatened a massive crackdown, either by Hong Kong’s police or by mainland troops.

The pro-democracy forces know they couldn’t fight martial law or an invasion, but they aren’t willing to give up their demands. Hence, their appearance Sunday outside the U.S. Consulate. They are hoping that the United States will employ its considerable leverage over Mr. Xi and Ms. Lam to deter the threatened repression.

Though it hasn’t been able to force Mr. Xi to make concessions on trade or stop fortifying islets in the South China Sea, the Trump administration has far-reaching influence over Hong Kong. More than 1,200 U.S. companies do business there, and 60 percent of foreign investment in China flows through the city, thanks to a U.S. law that designates it as a separate economic entity with the privileges of an open economy.

Unfortunately, President Trump has responded weakly to the protests and China’s threats. After weeks of praising Mr. Xi for tolerating “riots ” and expressing confidence that the Chinese dictator could quickly orchestrate a “happy and enlightened ending,” he reluctantly conceded late last month that a crackdown might make a trade deal impossible — not because of him, mind you, but because of “political sentiment.”

No wonder the demonstrators are appealing to Congress. They delivered a petition to the consulate calling for passage of the Hong Kong Human Rights and Democracy Act, which is pending in both the House and Senate. The measure would require an annual review of Hong Kong’s special economic status, and it would mandate sanctions on officials found to be suppressing “basic freedoms” in the territory. The sanctions might slow the recent wave of arrests of opposition leaders, while the reporting requirement signals that, as Sen. Marco Rubio (R-Fla.), a sponsor, says, the “nuclear option” of canceling Hong Kong’s special status will be on the table in the event of a larger crackdown.

The legislation appears to have bipartisan support: Both House Speaker Nancy Pelosi (D-Calif.) and Senate Majority Leader Mitch McConnell (R-Ky.) have spoken favorably about it. They should move swiftly to pass the bill, and Mr. Trump should sign it. Now is the time to send a clear message of deterrence to Beijing — and to show Hong Kong’s democracy movement that the United States is unambiguously on its side.

Read more: