Sam Rainsy says going to the polls will bring positive change in the country, including better working conditions, less corruption and improved public services.
The statement was sent to U.S. Secretary of State Rex Tillerson by Alan Lowenthal, Steve Chabot and Edward Royce.
Election observers and labor rights campaigners said without clear guidance many could be excluded from the results.
Sguon Samean is seen as “a courageous female commune chief” in Trapaing Chan commune, which is home to more than 6,000 villagers.
The Black Monday Campaign was created to advocate for the release of political prisoners in Cambodia.
The Overseas Press Club of Cambodia said in a statement that the move was “potentially dangerous and defamatory” and urged a retraction.
Rwanda marked the 23rd anniversary of the start of the genocide earlier last month and it went on for 100 days. VOA Khmer's Phy Sopheada narrates.
According to analysts, Hun Sen has not directly taken part in political campaigning since 1998.
Millions of Cambodians are due to take to the polls on June 4 to elect new local councilors and commune chiefs across more than 1,600 constituencies.
According to the National Election Committee (NEC), some 8,000 disabled people have registered to vote.
Prime Minister Hun Sen has regularly warned of “civil war” if the ruling party loses power.
The NDI is holding similar events in Takeo province’s Tropeang Thom Khang Cheung commune, on Wednesday, and Kratie province’s Svay Chreas commune, on Friday.