BANGKOK, Aug 22 (Reuters) - Thailand's Pheu Thai Party will seek to form a new government with some of its biggest rivals in a parliamentary vote on Tuesday, coinciding with the promise of a historic return from 17 years of exile by its fugitive figurehead Thaksin Shinawatra.

Thailand has been under a caretaker government since March and its new parliament has been deadlocked for weeks after anti-establishment election winners Move Forward were blocked by conservative lawmakers, leaving populist heavyweight Pheu Thai to lead a new effort.

The winner of five elections over the past two decades, Pheu Thai, a political juggernaut founded by the billionaire Shinawatra family, has agreed a contentious alliance including two parties backed by a military that overthrew two of its governments in coups in 2006 and 2014.

The lower house and military-appointed Senate must decide on Tuesday whether to endorse the prime ministerial nomination of Srettha Thavisin, a real estate tycoon who was thrust into politics just a few months ago.

Srettha said on Monday Pheu Thai had failed to secure the outright majority it had targeted, so its only chance of governing was in partnership with some rivals it had vowed not to work with.

"Things that were said during the election were one thing. But we didn't really get the landslide, so we had to renege," said Srettha, who has the support of 317 lawmakers and needs 58 votes from the Senate to secure the requisite backing of half of the legislature.

"We are not lying to the people, but we have to be realistic."