WASHINGTON, July 17 (Reuters) - The United States is concerned about developments in Thailand's legal system, State Department spokesperson Matthew Miller said on Monday, after two separate complaints against the leader of the party that won the most seats in a May 14 election.

Thailand's parliament is preparing a second vote on Wednesday on whether Pita Limjaroenrat, leader of the progressive Move Forward party, can become prime minister.

An initial vote last week for Pita - who wants to remove the military from politics and dismantle business monopolies, among other changes - was thwarted by a Senate appointed by the royalist military following a 2014 coup.

U.S. officials have said little about the post-election developments in Thailand, a longstanding military ally in a region where Washington is wary of China's growing influence.

Miller, asked at a regular press briefing about the situation in Thailand, said Washington does not have a preferred outcome in the Thai election, but supports a process that reflects the will of the Thai people.

"We are very closely watching the post-election developments - that includes the recent developments in the legal system, which are of concern," Miller said.

Thailand's Constitutional Court has accepted a complaint against Pita and Move Forward over a plan to change a law that prohibits insults against the royal family. The election commission has also recommended the same court disqualify Pita over ownership of shares in a media company in violation of electoral rules.

The cases have raised concerns the court could disqualify Pita from office or dissolve Move Forward, as it did in 2020 with the party's predecessor Future Forward.

Asked to comment on those possibilities, Miller said he would not "speculate about how we might react to events that have not yet occurred" but repeated that recent developments were of concern.