By Devjyot Ghoshal and Panu Wongcha-um
BANGKOK (Reuters) -Pita Limjaroenrat stood on the back of a truck, waving to hundreds of cheering supporters and signing autographs, a day after he led his Move Forward party to an extraordinary victory in Thailand's general election.
Behind him loomed a giant portrait of King Maha Vajiralongkorn, the head of a once seemingly untouchable institution that Move Forward has for the first time brought into public discourse in an election - a stance that may now complicate Pita's path to becoming prime minister.
There are other hurdles, including the ambitions of other opposition parties, an unelected Senate dominated by conservatives, and an accusation of wrongdoing against him, that analysts say could thwart Pita, despite Move Forward emerging as the single largest party in parliament after Sunday's vote.
Hours before Monday's victory parade, the youthful 42-year-old walked out in front of the press at party headquarters and declared: "I'm Pita Limjaroenrat, the next prime minister of Thailand."
Pita said he had spoken to five parties and proposed an alliance with 309 seats in the 500-member House of Representatives. But that is still short of the number needed to ensure he be elected prime minister, because a 250-member Senate appointed by a former military junta also votes for the top job.
Move Forward's main coalition partner would be the Pheu Thai party, backed by the billionaire Shinawatra family that was - until now - the main challenger of the conservative, military-backed royalist establishment that has held power for the last decade.
Pheu Thai, which won the last five general elections but got pushed out of power each time, secured 141 seats, according to the latest projections, only 10 fewer than Move Forward.
"It will not be a smooth transition," said Prajak Kongkirati, a political scientist at Bangkok's Thammasat University.
"There are many obstacles that Move Forward and Pheu Thai have to overcome."
Beyond divergent economic policies, the two differ in their approach towards the monarchy.
In opposition, Move Forward was the only party to scrutinise the royal budget, estimated at about 3.75 billion baht ($111 million). Pheu Thai steered clear of any such sensitive discussion.
Move Forward has promised to press ahead with a plan to amend a strict lese majeste laws against insulting the monarchy, which critics say have been used to stifle dissent.
Such positions make other parties uneasy about aligning with Pita but, so far, the opposition appear to be sticking together.
"Move Forward cannot take anything for granted," said Titipol Phakdeewanich, a professor of political science at Ubon Ratchathani University. "Pita has assumed they can form a government with Pheu Thai."
Born to a politically influential family, Pita attended high-school in New Zealand and holds graduate degrees from top U.S. universities including Harvard.
He cut his teeth working as a staff member to a minister in an administration led by then Prime Minister Thaksin Shinawatra, who founded the original incarnation of Pheu Thai.
Full-time politics came in 2019, when Pita entered parliament with 80 other members of the Future Forward Party.
Founded by Thanathorn Juangroongruangkit, a motor-parts billionaire, Future Forward stirred up politics with its demand for reform, including ending conscription and business monopolies.
But in February 2020, a court ordered the party dissolved and Thanathorn and 15 other party executives banned from politics for a decade for violation of election rules.
"The party is dissolved but the people's problems have not dissolved," Pita said at the time.
Remaining party members then formed Move Forward with Pita as leader.
Youth-led street protests erupted months later, as thousands called for reforms, including a new constitution to replace one drafted during military rule, amid unprecedented public criticism of the monarchy's influence on politics and society.
Some of those demands are reflected in Move Forward's platform.
"Move Forward takes the game to the next level with institutional reform," said Thitinan Pongsudhirak, who teaches political science at Chulalongkorn University in Bangkok.
"That's the new battleground."
Like his former party leader, Pita too faces the risk of disqualification.
Days before the election, a pro-establishment politician submitted a complaint alleging Pita held shares in a media company. Rules stipulate that an owner or shareholder of any mass media business cannot stand for election.
Pita has said the shares were part of his late father's estate, which he was tasked to manage.
"We will respond with maturity, legal principles as well as evidence," he told reporters. "I'm extremely confident."
The election commission is looking into the complaint.
To become prime minister, Pita must also navigate the upper house. Some of its 250 members have suggested they would not vote for Pita, raising the prospect of pro-military parties that got trounced in the election putting forward a candidate who becomes prime minister with the Senate's help.
That could trigger the return of the type of protests that have brought intermittent turmoil over the past two decades.
"It will be quite a hefty price to pay if someone is thinking about debunking the election result or forming a minority government," Pita said.
"I'm not worried but I'm not careless."
(Reporting by Panu Wongcha-um and Devjyot Ghoshal; Editing by Robert Birsel)