FOR NEARLY two decades, politics in Thailand has been polarized between a rural-based populist movement and an urban elite allied with the military and monarchy. The populists repeatedly won democratic elections, only to be ousted by coups and questionable court decisions. Since 2014, when a coup installed its military leader, Prayuth Chan-ocha, as prime minister, the reactionaries have held the upper hand.

Now a new force has appeared that could shift the balance of power. A student movement that began months ago with protests against educational practices has gradually blossomed into a broad pro-democracy movement. Thousands have marched in Bangkok to advance three demands: that Mr. Prayuth and his government step down, that a repressive constitution imposed by his regime be revised and that government critics be allowed to speak freely. More recently, the protesters have been calling for reforms of the powerful monarchy, reacting to growing abuses by the current king.

Last week, the regime attempted to quash protests by declaring a state of emergency in Bangkok, banning gatherings and deploying water cannons against the students. Dozens of protest leaders have been arrested. Yet over the weekend, thousands defiantly gathered for new demonstrations. Employing tactics of the protest movement in Hong Kong, they used secure messaging apps to arrange flash demonstrations that police were unable to contain. Meanwhile, unrest began spreading to areas outside the capital, where the “red shirt” populist movement once led by Thaksin Shinawatra remains strong.

The developments offer a ray of hope that democracy in Thailand could be revived after years of repression. Oddly, one factor on the side of the reformers is the behavior of King Vajiralongkorn, who replaced his beloved father in 2016 and has alienated much of the country with his eccentric and autocratic behavior. A notorious playboy who spends most of his time in Germany with a large entourage of servants, the king has sought to expand his powers beyond his status as a constitutional monarch, seizing control of billions of royal assets as well as military units. Those who criticize him risk long jail sentences under lèse-majesté laws: Two protesters who yelled at the royal Rolls-Royce limousine as it passed through the capital last week have been charged with endangering the royal family, which could mean a sentence of life in prison.

The protesters will surely persist, but they face an uphill battle. China seems to have shown in Hong Kong how a street protest movement can be gradually ground down, even without a Tiananmen Square-style crackdown. In the past, a pro-democracy movement might have looked to the United States for help; Washington’s strong military alliance with Thailand provides it with leverage. But, as in so many other nations where opponents of autocracy struggle to gain traction, the Trump administration has been silent. No doubt Mr. Prayuth sees that as a vote of confidence, and a green light for more repression.

Read more: