The takeaway from Thailand’s general election in May was clear: Voters had dealt a crushing blow to the ruling military junta by supporting a progressive party that challenged not only the generals but also the nation’s powerful monarchy.

The generals and their allies responded on Thursday by rejecting the party’s leading candidate for prime minister, tipping the country into a political void and potentially thrusting it further toward autocracy.

Parliament failed to elect a new prime minister on Thursday evening after the progressive candidate, Pita Limjaroenrat, was unable to muster enough support in the military-backed Senate, where lawmakers are loyal to the generals who have governed Thailand since seizing power in a coup nearly decade ago.

As night fell over a rainy Bangkok, one of Southeast Asia’s most important economies was staring down what looked like another intense period of political unrest and nationwide protests.

“This is déjà vu,” said Thitinan Pongsudhirak, a political science professor at Chulalongkorn University, referring to the cycles of elections, protests, coups and crackdowns that have occurred in Thailand since 2007.

Now it is up to Parliament to pick from the field of candidates again, through what is likely to be a tumultuous week ahead that may or may not end with a new prime minister in charge. A second vote is scheduled for July 19. A third, if necessary, would be held a day later.

While Mr. Pita, 42, is relatively new to Thailand’s political drama, the queasy feeling of drifting toward civil strife is not. The country’s recent history is littered with military coups; protesters have led widespread demonstrations against a royalist establishment that they say has consistently thwarted efforts to introduce democratic reforms.

“There’s a pattern here of establishment pushback against any progressive movement in Thai politics,” Mr. Thitinan added. “And the pushback comes in different shapes and forms,” including dissolutions of political parties and disqualifications of major candidates.

Ahead of the vote on Thursday, Mr. Pita, a former technology executive who holds graduate degrees from prestigious American universities, had positioned himself as a champion of reform. On the campaign trail he called for amending a law that criminalizes public criticism of the Thai monarchy — a move considered unthinkable a decade ago.

“I want to be the leader of the people,” he said in Parliament on Thursday. “To tell the world that Thailand is ready. To look for a new balance between international political powers.”

But Thailand’s Parliament appeared unwilling to embrace such a vision. Even though Mr. Pita’s political party, Move Forward, had built a multiparty coalition, he received only 324 combined votes in the House of Representatives and the Senate — short of the 376 he needed to win the premiership.

Supporters of Mr. Pita’s coalition had gathered on Thursday outside the parliament building in Bangkok where the vote was held, and some had vowed to hit the streets in protest if he did not win enough votes to become prime minister.

“The votes that have been cast, the 25 million votes, are sacred voices that will shape the future of the country,” Arnon Nampha, a political activist and protest leader, said during a protest on Wednesday night, referring to the votes in May for Move Forward and Pheu Thai, the second-largest party in the coalition.

“If you want to change this, no way, we will not allow it,” he added.

Mr. Thitinan said he expected a reprise of the flash mob-style protests that erupted in Thailand during the coronavirus pandemic and were led by young demonstrators calling for checks on the Thai monarchy’s vast power.

Mr. Pita had already been dealt a major setback on Wednesday when Thailand’s Election Commission asked the Constitutional Court to suspend him from Parliament. He had been under investigation for allegedly owning undeclared shares in a media company, which could disqualify him from running for office.

The Constitutional Court also said on Wednesday that it had accepted a complaint against Mr. Pita over his calls to amend the law that penalizes criticism of the monarchy. Analysts predicted that both moves would give Mr. Pita’s opponents in the Senate a convenient excuse not to vote for him.

Mr. Pita’s progressive coalition may not be strong enough to weather the loss. Members of Pheu Thai, in particular, could try to form a new coalition that is led by one of its own candidates for prime minister.

A likely scenario is that Pheu Thai would field Srettha Thavisin, a property tycoon who is considered a more palatable candidate among Thailand’s military establishment. Military-backed lawmakers may vote for Mr. Srettha, said Wanwichit Boonprong, a political scientist at Rangsit University, outside Bangkok.

Still, he said, Pheu Thai could be a good compromise for reform-minded voters who had supported Mr. Pita.

As for the old guard, Prime Minister Prayuth Chan-ocha, the general who took power after leading Thailand’s 2014 military coup, said on Tuesday that he would retire from politics once a new government is formed. But even if he does retire, analysts said the military and its allies may try to hold onto power in other ways.

The military has engineered a system in which it essentially controls one chamber of the legislature, the Senate. To keep one of its own in charge, the military could promote Gen. Prawit Wongsuwan, a member of the ruling party, as a possible candidate for prime minister during the vote next week.

“Almost all the senators were handpicked by General Prawit,” said Jade Donavanik, an expert on Thai politics at the College of Asian Scholars in Thailand, referring to the 250 members of that chamber. “This is part of the problem.”

The election is being closely watched, not least because Thailand is a major player in a region where several countries have been sliding again toward autocracy after experiments with democracy. Thailand was once a stable ally of the United States but has moved closer to China under the current junta.

For decades, the country was dominated by two opposing political forces — one led by conservative royalists and militarists, the other by Thaksin Shinawatra, a former telecommunications tycoon and populist politician who served as prime minister for five years before he was ousted in a 2006 coup.

His sister Yingluck Shinawatra became prime minister 2011 and was forced from office days before the 2014 coup.

Move Forward has captured a similar sort of energy that Mr. Thaksin’s populist movement once did, and its failure on Thursday appeared to be another example of Thailand’s royalist establishment snuffing out a popular political candidate.

Mr. Wanwichit, political scientist at Rangsit University, said that Move Forward’s aggressive calls for reforming the monarchy may have been too extreme for most voters, even those who consider themselves liberal and in favor of democratic reform.

“For now, the monarchy is seen as the main pillar of the country,” he said. “Whether you are liberal or conservative, you still respect the monarchy as embodying the dignity of the nation.”