POPULAR DEMONSTRATIONS against democracy are becoming an unfortunate trend in developing countries where elections have challenged long-established elites. The latest case is Thailand, where thousands of people took to the streets Monday to demand that the country’s freely chosen government step down, that an unelected council take its place and that elections scheduled for next month be canceled. The protesters’ strategy appears to be to disrupt Bangkok to the point at which the government will feel compelled to resign or be removed by the military.

Similar tactics have succeeded in bringing down two previous governments led by former prime minister Thaksin Shinawatra and his supporters since 2006, while a third was forced out by a dubious court decision. This time, Prime Minister Yingluck Shinawatra, Mr. Thaksin’s sister, is standing firm, as she should. But she could use more support from the United States in rejecting an undemocratic outcome to the crisis.

Ironically, what amounts to a coup attempt is being supported by many in the opposition Democratic Party. The party’s problem with democracy stems from its failure to win an election since 2001, when Mr. Thaksin’s movement first surged to power. The Democrats represent Bangkok’s middle and upper classes and the traditional business establishment linked to the royal family. Mr. Thaksin, a billionaire businessman who now lives in exile, is a populist who draws support from previously disenfranchised Thais — the poor in urban areas and the rural northeast. While he did abuse power and commit human rights abuses when he was in office, his governments were freely and fairly elected, as was Ms. Yingluck’s.

In a decade of political and street conflicts, the Thaksin movement has seemed to mature: Ms. Ying­luck’s administration was proceeding relatively smoothly until she attempted to pass an amnesty bill through parliament that would have allowed her brother to return home. The opposition, meanwhile, has grown more radical. No longer do its leaders claim, as they once did, to be liberal democrats who seek only to correct Mr. Thaksin’s abuses. Now they aim explicitly at installing a regime that would empower a minority while seeking the “eradication” from politics of Mr. Thaksin and his family.

Opposing such an agenda ought to be an easy call for the United States, which has close economic and security relations with Thailand. But as was the case when Egyptians sought to provoke a coup against their elected government last summer, the Obama administration’s response has been weak. A State Department spokeswoman called Monday for the crisis to be resolved through a “democratic process” and praised the government’s “restraint” in responding to the demonstrators.

The administration has not, however, made clear publicly that a coup — whether by the military or the street mobs — would be unacceptable to the United States or that it would result in a suspension of aid and security cooperation. U.S. law mandates such a cutoff, but since the administration declined to observe the statute following Egypt’s military coup in July, Thailand’s anti-democracy militants may be emboldened to believe that they, too, will be tolerated by the Obama administration. They shouldn’t be. As has been the case in Cairo, the victory of the anti-democracy forces would only lead to more violence and instability.