BANGKOK — Thailand’s Constitutional Court on Thursday banned a political party that nominated a princess as its candidate for prime minister, a major blow to a movement aligned with ousted prime minister Thaksin Shinawatra ahead of crucial elections.

The court said the party, Thai Raksa Chart, violated the law by choosing to involve a royal — in this case the king’s elder sister, Princess Ubolratana — in its campaign, breaking a long-standing tradition that places the palace above politics. The nine judges on the court voted unanimously for the dissolution. 

“Though Thai Raksa Chart had obtained political rights and freedom granted by the Thai constitution, exercising that right must base on an awareness that it will not destroy Thai norms, values and free will,” said Nakharin Mektrairat, a judge on the court. “If the royal family is pulled down to politics, the royal institution will not have its status as the center for Thai people anymore. This will make constitutional monarchy decline and end.”

The party’s executive members will also face a 10-year ban on involvement in politics. 

The ruling had been widely seen as inevitable, after Thailand’s King Vajiralongkorn issued a rare royal order just hours after his sister’s candidacy was announced Feb. 8, calling the move “extremely inappropriate.” In Thailand, the monarchy occupies a revered place in society, and criticism of the monarch is a criminal offense under harsh lèse-majesté laws. 

Replying to a supporter on her popular Instagram page after the verdict, Ubolratana called the dissolution of the party “sad and depressing.”

Still, supporters of the Thaksin-linked party, as well as legal experts, have in recent weeks rejected the idea that Thai Raksa Chart’s move was against Thai law. The ruling has opened up familiar divisions in Thai politics, reminiscent of previous moves to thwart Thaksin’s populist movement, and has raised the specter of tensions bubbling over after the March 24 vote. 

“In the past 10 years, the dissolution of political parties in Thailand and whether it was based on rule of law has been questioned,” said Sunai Phasuk, a senior researcher at Human Rights Watch in Thailand. The case of Thai Raksa Chart “brings back that question again.” 

Rights activists pointed out that the election commission, which referred the case, took only a day to make its decision that the party should be dissolved. The Constitutional Court did not have a full trial before making its ruling. 

Speaking to dozens of journalists gathered at the court, Thai Raksa Chart leader Preechapol Pongpanich said his party felt “deeply sorry” about the decision. 

“This affects the basic rights of the party’s candidates,” he said in brief comments, his voice shaking. “Unfortunately, our party could live only four months. Though we can’t reach our goal on March 24, we still want to thank our supporters.”

The party argued to the court that it was Ubolratana’s decision to run for prime minister. On her Instagram page, she has said that she merely wanted to work for the good of the people and that her candidacy should not have caused “problems.” Ubolratana gave up her royal titles in 1972 when she married an American, but upon their divorce she returned to Thailand in 2001, resumed royal duties and ran a youth-focused charity.

Ahead of the verdict, Thai police deployed at least 1,000 security officials and several large police trucks to the court. Checkpoints were set up in the surrounding area, and police banned public gatherings within 165 feet of the Constitutional Court and the Supreme Court. 

“As usual, we always deploy police in every high-profile political case. It’s a normal operation,” said Chayapol Chatchaidet, an official from the Special Branch Bureau of the Thai police.

The elections this month will be the first held since 2011, and the first since the military pushed out Yingluck Shinawatra, Thaksin’s sister, as prime minister in a 2014 coup. Thaksin and his allies have won every election since 2001, but he and his sister live outside the country as fugitives after fleeing corruption charges. 

The vote will pit supporters of the Thaksin movement and other newer parties that oppose the military government against parties aligned with the junta and Prime Minister Prayuth Chan-ocha, who is seeking to keep his post. 

Thaksin’s main party, Pheu Thai, has faced the threat of dissolution for some time. It also has been affected by new rules put in place by the military junta ahead of elections to prevent any one party from winning a landslide. 

Parties such as Thai Raksa Chart were set up as insurance options in a strategy to allow Thaksin’s allies to control a greater number of seats in parliament. Thai Raksa Chart was contesting 175 constituencies out of the 350 available, and it will lose its candidates in each one. 

“It is a problem,” Pheu Thai leader Sudarat Keyuraphan said in an interview ahead of the Constitutional Court’s decision. But, she added, “if there isn’t Thai Raksa Chart, the vote [of those who support it] will go back to another party on our side.”

Despite their ouster, Thaksin and his allies still loom large in Thai politics, seen as the only politicians who can speak to the struggles of the rural poor. When the Constitutional Court removed Yingluck as prime minister in May 2014, thousands of supporters gathered at the court in protest. In this year’s election campaign, Pheu Thai rallies have continued to draw thousands of supporters across the country.

Many analysts say the election will raise more questions than it answers. The vote is likely to leave political gridlock between the military and popular elected politicians, without resolving the ongoing question of where the monarchy should lie between them. 

“The election is likely to lead to dead ends in governance and an eventual crisis,” said Thitinan Pongsudhirak, a political scientist at Chulalongkorn University. “If compromise is not in the cards, then we can expect a tailspin that leaves Thailand unsettled for a long time.”

Mahtani reported from Hong Kong.