Thailand's two main opposition parties won big in the general election and have agreed to seek a ruling coalition to replace the current military-backed government.
The Move Forward Party is led by 42-year-old businessman Pita Limjaroenrat.
It placed first in the Sunday voting.
In second place was the Pheu Thai party, which had been favored to win.
Pheu Thai is led by Paetongtarn Shinawatra, the daughter of former Prime Minister Thaksin Shinawatra.
Thaksin was ousted by a military coup in 2006.
He has lived in exile since being removed from power.
But he remains the main driving force behind the Pheu Thai party.
Election results showed the Move Forward and Pheu Thai parties won a majority of seats in the House of Representatives.
The two parties are mostly allied in their opposition to the military's past interference in politics.
There have been numerous coups since 1932, the year Thailand became a constitutional monarchy.
Both opposition parties also favor reforms of some parts of the monarchy.
Pita said Monday he had been speaking with a total of five parties in an effort to form a coalition government.
The prime minister is elected by a joint session of parliament.
That vote includes 500 members of the newly elected House and 250 senators.
Pita's proposed alliance would have 309 House seats.
But that falls short of the 376 votes needed to ensure he is elected prime minister.
Parliament chooses a new prime minister in July, so negotiators will have about two months to reach an agreement.
The 250-seat Senate has been supportive of the country's military and courts.
Its members voted unanimously in 2019 to elect current Prime Minister Prayuth Chan-ocha.
Prayuth, a former army commander, first came to power in a 2014 coup.
The 2019 Senate vote enabled him to return as prime minister even though Pheu Thai candidates finished first in that election.
Prayuth ran for reelection on Sunday.
But he has not enjoyed wide support because many people blamed him for continuing national problems.
These included weak economic policies, mistakes dealing with the COVID-19 pandemic and blocks of democratic reforms.
Pita was asked by reporters about how a future vote could go in the Senate.
He said all sides must respect the election results and there was no use trying to change the established parliamentary process.
"I am not worried, but I am not careless," Pita said.
Move Forward's supporters drove through the streets of Bangkok Monday to celebrate their victory.
Pita smiled and waved to supporters as vehicles carried him and other party members through the streets of the old part of the capital.
Pita said, "Today it's a new day, and hopefully it's full of bright sunshine of hope going forward."
Earlier, as it became clear his party was taking the lead, he tweeted that he is prepared to bring about change as the country's 30th prime minister.
"Whether you agree or disagree with me, I will be your prime minister. Whether you have voted for me or not, I will serve you," Pita said.
I'm Bryan Lynn.