BANGKOK -- Thai Prime Minister Prayuth Chan-ocha's political survival skills are being severely tested early into his second term in office, as the head of a shaky coalition is facing a series of difficulties.

The current issues surrounding the prime minister pose a serious threat to his sprawling coalition of 19 parties, which only has a slim majority in the country's lower house of parliament.

At issue for the prime minister's opposition is the political fitness of Thammanat Prompao, deputy agriculture and cooperatives minister, a key ally in Prayuth's cabinet. He was recently revealed in the Australian press as having been jailed in the 1990s for heroin trafficking in that country, to which he has responded with denials and threats of libel suits.

Thammanat was previously credited with roping in 10 small political parties to join Prayuth's coalition and ensure its slender majority following an unclear election result earlier this year. The politician, who is known for speaking his mind, has likened his job to that of a "monkey keeper" feeding bananas to members of these 10 parties in order to keep them in line -- a comment that has incensed many of those to whom he referred.

Since details of Thammanat's past were reported in The Sydney Morning Herald, Prayuth and leading coalition ministers have closed ranks behind him. Other questions have been raised over Thammanat's time in the Thai army and the authenticity of his doctoral degree. A member of the country's former ruling junta, which was headed by Prayuth, has also called him one of 6,000 "influential criminal figures."

According to Sunai Phasuk, senior Thai researcher for campaign group Human Rights Watch, Thammanat's parliamentary appointment has caught the attention of the local and international community because of his notoriety.

"Now he faces accusations of lying during the parliamentary inquiry about his criminal record; this is a big deal," said Sunai, referring to a clause in the constitution preventing Thais convicted of drug offenses from becoming parliamentarians. "It also damages Prayuth, who keeps saying he has cleaner politicians on board, unlike other previous governments."

But Prayuth had a preexisting problem, which emerged at the end of the first parliamentary session since the March general election. Controversy over his swearing of an oath to the constitution has been used by the opposition to question the legitimacy of Prayuth's government.

Thailand's Deputy Agriculture and Cooperatives Minister Thammanat Prompao talks to reporters after a government cabinet meeting in Bangkok on Sept. 10. © Reuters

In mid-July, he failed to recite the entire oath of office before the Thai king -- the omission being the final sentence: that his government would pledge to "uphold and observe the Constitution of the Kingdom of Thailand in every respect."

Opposition parliamentarians have pressed him on this lapse, since a failure to uphold the charter makes his government illegitimate in their eyes, but during a brief appearance in parliament on Sept. 18, Prayuth dodged the illegitimacy question.

Complicating matters further, critics have also asked Prayuth whether he was ordered to omit this section, putting him in a tight spot over a political taboo: to admit to involvement by the royal palace. The country has severe lese majeste laws to protect the king and selected members of the royal family from insult, threat or defamation. Any act of defamation carries a jail term of three to 15 years.

Political insiders believe that Prayuth is counting on being thrown a lifeline by the supposedly independent Constitutional Court to help him out of this political morass. On the eve of his showdown with the parliamentary opposition this week, the court rejected a petition to rule on the issue, saying that the oath-taking issue "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."

Thus far, Prayuth's Teflon coating seems to have endured, but Bangkok-based diplomats say it might start to flake in the face of enduring criticism. His critics are asking whether his government is disregarding Thailand's claims to be a constitutional monarchy, ignoring the law and aiming to function above the constitution, one diplomat said.

"Prayuth finds himself at the heart of the future shape of the Thai state and its power structure," the diplomat said.

Political commentators believe that the current challenges will test the prime minister's strength to retain his slender majority in the 500-member lower house. It begins its second session later this year with voting on the budget for fiscal 2020 -- the first major political test to face his 19-party coalition.

These first brushes with a parliamentary opposition are new to Prayuth. He ruled unopposed after he led a coup in May 2014 to topple an elected government until the elections this March.

This new reality is setting in even in the prime minister's office. "The mood has changed after five years of a military government with bureaucrats and officers managing things," said a well-placed source in the office. "Now the politicians have taken over, it has become fickle and treacherous."

Observers note that some challenges have come from within the coalition's ranks, with some cabinet ministers who were elected to parliament in March breaking with political tradition and not giving up their parliamentary seats in order to hold ministerial posts.

"Prayuth has little leverage to force them to give up now," said one observer. "This can become a problem during a vote in the house when [certain MPs will be] away on [their] business as a minister."

The uncertainty spreads beyond the halls of parliament. The annual reshuffle of generals in Thailand's powerful military, which has just been approved by the king, marks a clear break with an influential faction within the army, the Queen's Guard. Prayuth rose through the ranks of this elite unit, as did four other former army chiefs -- all of whom played leading roles in the 2014 coup and the five years of military rule.

There is now no Queen's Guard faction member among the "Five Tigers" -- the top five command roles in the army -- said Paul Chambers, an expert on Thai national security at Naresuan University, located in the north of the country. "This is the first time this has happened since 2003."