BANGKOK, Aug 21 (Reuters) - Thailand's Pheu Thai Party will attempt to form a government on Tuesday after weeks of post-election deadlock, with real estate tycoon Srettha Thavisin set to be nominated as prime minister and face a vote in parliament.

WHO IS SRETTHA?

Srettha, 60, is an outsider and political neophyte little known beyond Thailand. He joined the populist Pheu Thai in 2022 and up until the May 14 election was president of property developer Sansiri. He is not a member of parliament.

The six feet, 3 inch (1.92 m) tall Srettha is a finance graduate from the Claremont Graduate School in the United States who once worked for Proctor & Gamble in Thailand.

Though he has no experience in government, his political ascent has been largely welcomed by businesses and he is untainted by the bitter power struggles that have dogged Thai politics for nearly two decades.

WHY IS HE BEING NOMINATED FOR PM?

Pheu Thai came a close second in the election but its pact with the winner, the progressive Move Forward, fell through after that party's prime ministerial candidate Pita Limjaroenrat failed to win support from lawmakers allied with the royalist military. Pheu Thai is now leading the effort to form a government.

The billionaire Shinawatra family that founded Pheu Thai has a long and bitter history with the military, which ousted the governments of Thaksin Shinawatra and his sister, Yingluck Shinawatra, in 2006 and 2014 respectively.

Thaksin's youngest daughter, Paetongtarn, was another possible candidate for prime minister. But to Pheu Thai's rivals, newcomer Srettha could be a more palatable, compromise than having another Shinawatra at the helm.

DOES SRETTHA HAVE PARLIAMENT'S BACKING?

Fourteen parties have pledged support for Srettha and Pheu Thai in the lower house, but the biggest stumbling block will be the upper house Senate, whose 249 members were appointed by the generals who toppled Pheu Thai's last government.

The military created the rules under which a prime minister is selected, with the Senate essentially protecting establishment interests by serving as a bulwark against elected progressive politicians.

The Senate's largely conservative members were instrumental in ensuring Move Forward's Pita failed and it is unclear how many will see Srettha and Pheu Thai as a better option.

Srettha needs 375 votes, or more than half of the combined lower and upper houses. Pheu Thai's alliance has 11 parties with a combined 314 seats, and three more lower-house lawmakers backing it, so it needs the votes of 58 Senators.

WHAT'S DIFFERENT THIS TIME?

The military-backed United Thai Nation and Palang Pracharat parties that were instrumental in thwarting Move Forward have joined the alliance backing Srettha.

But scepticism remains about how firm that support is, given the army's years-long efforts to undermine Pheu Thai. To have such backing from its enemies indicates some kind of once unthinkable power-sharing agreement may have been made behind the scenes.

That would boost Srettha's chances of winning the backing of Senators allied with the army.

However, given the fraught history, there will be doubts about the effectiveness of such a coalition, how long a Pheu Thai-led government might last, and whether the military and allies in key institutions might try to push Pheu Thai out later.

An opinion poll published on the weekend showed many Thais were not happy with the idea of a Pheu Thai-military pact.

WHAT ROLE IS THAKSIN SHINAWATRA PLAYING?

Officially, none, but as the self-exiled figurehead of the Pheu Thai political juggernaut, it is almost certain Thaksin would have been involved in any deal made between rival camps.

Thaksin, 74, is a fugitive in Thailand and plans to return on Tuesday, the same day as the vote, after 17 years abroad dodging jail sentences handed down in absentia for abuse of power and more.

The change of heart suggests Thaksin is confident the vote will go in Pheu Thai's way and any deal he has with old enemies will hold - including over his incarceration - despite the history of mistrust and betrayal. Some people in Thailand, however, are sceptical about Thaksin's promised homecoming and see it as political theatre. (Editing by Robert Birsel)