BANGKOK, Aug 21 (Reuters) - Thailand's Pheu Thai Party said on Monday its bid to lead the next government had the backing of 14 parties, including two allied with its military rivals, a day ahead of a parliamentary vote for a new prime minister.

Thailand's bicameral parliament has been deadlocked for weeks on the formation of a new government after a May 14 election dominated by two former opposition parties, winners Move Forward and second placed Pheu Thai.

The two parties trounced conservative, pro-army opponents in what was seen as a resounding public rejection of nine years of government led or backed by the royalist military, which ousted Pheu Thai governments in 2006 and 2014.

Real estate mogul Srettha Thavisin of the populist Pheu Thai had the backing of 14 parties to become prime minister in Tuesday's parliamentary vote, according to Phumtham Wechayachai, the party's deputy leader.

The party has taken over the formation of government after a bid by the progressive Move Forward failed last month, demonstrating fierce conservative resistance to its anti-establishment agenda.

Move Forward has said it would not back Pheu Thai's multi-party effort, arguing it distorted the election result and was against the public will.

Srettha, 60, a political newcomer, needs the support of 375 lawmakers, or more than half of the combined upper and lower houses of parliament, to be endorsed as premier and form the next government.

If successful, that would include winning over dozens of legislators allied with the military, in a fragile pact between erstwhile foes that could prolong concerns of a new bout of political instability after more than 15 years of on-off turmoil.