PHNOM PENH, Cambodia — It’s election season in Cambodia, and the fireflies are out.

Cambodians use that term — “ampil ampik,” in the Khmer language — to refer to little-known political parties that flash onto the scene shortly before an election, then fade back into obscurity.

Twenty parties, some just a few months old, will be on the ballot when national elections are held this month. But most voters will have heard of only one: the Cambodian People’s Party, led by Hun Sen, the authoritarian prime minister.

Mr. Hun Sen has had no viable opposition since November, when the Cambodia National Rescue Party — which almost won the 2013 election — was dissolved by a court packed with his loyalists. The United Nations special rapporteur on Cambodia, Rhona Smith, and numerous rights groups have said the July 29 vote will not be legitimate.

In response, the government points to such obscure entities as the Dharmacracy Party, the Khmer Will Party and the New Light Party (whose platform is to promote “Cambodia’s natural, linguistic and alphabetical wonders”).