US Ambassador Joseph Mussomeli expects a peaceful transition of power following free and fair elections in July.
Critics maintain that the ruling party continues to buy off or otherwise persuade activists from competing parties and to buy votes through gifts to the poor.
Mussomeli told VOA Khmer in Washington the election was likely to be peaceful and orderly.
That may be true, but the ruling party continues to buy off activists and buy votes through gifts to the poor, Kem Sokha, president of the Human Rights Party said.
"The ruling party is buying or persuading the other political parties’ activists or offering gifts to poor people in the rural countryside in order to get their ballots,” he said. "Since my Human Rights Party was established, I've received so many threatening cases and more than 200 party signs were abolished."
Om Yintieng, a senior advisor to Prime Minister Hun Sen, said Cambodia's political environment had gotten "better and better" since elections in 1993.
"I don't think it can satisfy everybody in the world, but we are proud for our country to be able to improve the election process," he said.
Koul Panha, executive director of the Committee for Free and Fair Elections, said he was concerned for the election process, which has already seen eight murders and 20 cases of intimidation.