BANGKOK, Thailand — It’s a rare case of a politician getting into trouble for something he didn’t say.

When Prayuth Chan-ocha, Thailand’s authoritarian prime minister, was being sworn into office in July after winning an election his critics say he engineered in his favor, he donned an immaculate white uniform and began reciting the required oath of office during a live televised ceremony in front of the king.

Mr. Prayuth, who first came to power in a military coup five years ago, clearly recited the oath’s first words, reading aloud, “I, Prayuth Chan-ocha, swear I will be loyal to his majesty and perform my duties honestly for the benefit of the country and the people.”

But the prime minister-elect suddenly stopped, failing to recite the oath’s final line: “I will also uphold and comply with the constitution of the kingdom in every aspect.”

His sudden and confusing silence — still unexplained two months later — has morphed into a genuine political controversy, plunging this dangerously polarized country into a swirling, unresolved debate and raising questions about why the prime minister and his Cabinet held their tongue.

While opposition leaders and the press have questioned the conspicuous omission, Thailand’s powerful Constitutional Court on Wednesday rejected a legal challenge to Mr. Prayuth springing from a complaint filed by two citizen-activists to the independent Office of the Ombudsman. The office concluded last month that the prime minister breached the constitution by failing to recite the full oath.

But the court held that the legitimacy of the inauguration ceremony was a matter for the prime minister and the king to decide.

“The oath-taking concerns an action which reflects a ‘specific relationship’ between the cabinet and the king, and is considered a political issue under an act of government,” the court said in its ruling. “The court thus cannot accept the petition for consideration.”

The court’s opinion is significant, for it has toppled elected governments in the past for violating the old Thai constitution, a constitution Mr. Prayuth rewrote and replaced in a 2016 national referendum.

The court said it was “not in its authority” to rule on Mr. Prayuth’s failure to recite the entire oath on July 16 before King Maha Vajiralongkorn in the elegant Dusit Palace.

But opposition leaders in the Thai national assembly say the court’s washing of its hands in the controversy only opens the way for lawmakers to debate what to do.

“Now that the court has dismissed the petition, the debate can proceed without restrictions,” said parliament’s opposition chief whip Suthin Khlangsaeng on Wednesday.

“We will be able to scrutinize the matter, and the prime minister now has no excuse to avoid” answering questions, said Mr. Suthin, a member of the influential Pheu Thai (“For Thais”) party.

Compounding the confusion

Mr. Prayuth’s inscrutable response has only compounded the confusion. He has insisted he will not quit over the botched oath-taking, but has cryptically suggested he made a mistake.

“I would like to apologize to all ministers,” the prime minister said last month even after declining to comment on the growing controversy. “I take sole responsibility,” he said without elaborating during a speech describing his new policies to 800 government officials.

“It will be settled soon, because I had no intention of doing it wrongly,” Mr. Prayuth said a day earlier. “Let us focus on the intention.”

He has also warned opposition parties not to blow the oath controversy out of proportion.

The oath furor is all the more mysterious as it comes under a constitution Mr. Prayuth took a heavy hand in re-writing. The former military general seized power in a 2014 coup, tearing up the nation’s previous constitution in the process.

He orchestrated the campaign to draft and approve a fresh charter as head of a military-dominated junta in Bangkok, strengthening the military’s political domination but also maintaining Section 161, which required the new prime minister and Cabinet ministers to swear the oath to the king and vow to obey the constitution.

Mr. Prayuth has spent the past two months angrily rebuffing demands that he explain why he read out only part of his official vow, and recite the oath again in its complete form.

In Thailand, a new leader does not repeat the vow line-for-line as read out by a judge. Instead, the prime minister holds a large note card and reads the oath’s short text while presenting his Cabinet to the king.

Some opposition politicians, local media, academics and others publicly wondered if Mr. Prayuth does not want to obey the constitution after a nationwide parliamentary poll in March transformed him from head of a coup-installed junta to an elected leader.

Others speculated Mr. Prayuth made an embarrassing mistake, which he is even more embarrassed to correct.

“Prayuth Not Quitting for Botching Oath,” said the headline of Khaosod English news media. “Oath Blunder” and “Oath Gaffe,” said the headlines in the Bangkok Post.

“If he lets the oath-taking remain incomplete with an important part missing, future prime ministers may make up their own oaths without complying with the charter,” parliamentarian Piyabutr Saengkanokkul, secretary-general of the opposition Future Forward Party.

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