Thailand’s election authorities are seeking to disband a leading opposition party for nominating a princess as its candidate for prime minister—strengthening the ruling junta’s hand as next month’s pivotal national election approaches.

The Election Commission’s decision Wednesday to take the matter to the country’s constitutional court follows Friday’s extraordinary sequence of events, in which the announcement of Princess Ubolratana’s candidacy was followed quickly by King Maha Vajiralongkorn’s declaration that he wouldn’t...