Lawmakers called Tuesday for the United States to cut off aid to Cambodia unless veteran strongman Hun Sen allows free elections on July 28.
Hun Sen, who has ruled Cambodia for 28 years and vowed to stay in power for another decade, has been accused of muzzling dissent to eliminate any chance of losing.
The main opposition leader, Sam Rainsy, lives in exile and faces prison if he returns as planned. Cambodia briefly banned foreign-produced radio broadcasts ahead of the election.
Representative Steve Chabot, who heads the House Foreign Affairs subcommittee on East Asia, said he had no doubt Hun Sen would win a fresh term as prime minister "through the incitement of political violence, corruption and nepotism."
Chabot praised the decision of US President Barack Obama and other Western governments not to send observers to "legitimize" the election.
"But declaring the election 'not free or fair' is not enough," Chabot, a member of the rival Republican Party, said at a congressional hearing.
"US policy toward Cambodia needs to change and the Obama administration needs to take a much tougher approach to Asia's longest-ruling dictator," Chabot said.
Chabot said he was introducing legislation to cut assistance to Cambodia if the election is not fair.
Two prominent Senate Republicans, Marco Rubio and Lindsey Graham, introduced a similar bill saying that a Cambodian government formed through "illegitimate elections" would be ineligible for "direct" US aid.
The proposal also called for the United States to reduce development assistance and to use its influence to encourage international institutions to do likewise.
Obama has made Southeast Asia a top priority but has kept a distance from Hun Sen. During a visit to Cambodia in November to attend an East Asia Summit, Obama pressed Hun Sen on human rights and democracy in a meeting that the White House described as tense.
The administration has nonetheless requested $73 million for Cambodia in the next fiscal year, almost the same as in previous years.
Cambodia could also stand to gain under a $50 million initiative, launched by former secretary of state Hillary Clinton, to support health, women's rights and the environment in the Lower Mekong region.
The amount is tiny compared with billions of dollars in investment in infrastructure and other areas promised by China, which has increasingly found an ally in Hun Sen.
John Sifton, the Asia advocacy director of Human Rights Watch, told the hearing that the United States nonetheless had clout. He said that the ruling Cambodian People's Party has released campaign material that shows Hun Sen next to Obama during last year's summit.
Sifton said that the United States should cut ties to the Cambodian military and speak out "loudly" after the election to make clear "that one-man and one-party rule are not acceptable in the 21st century."
More than 85 percent of US aid to Cambodia goes to development or health, including fighting AIDS, malaria and other diseases in a country that suffered some of the 20th century's worst horrors under the Khmer Rouge regime.
The United States routes much of its aid through non-governmental organizations, a policy required by Congress until 2007.
Rainsy, who lives in exile in France, told AFP during a May visit to Washington that the United States should consider economic sanctions if Hun Sen goes ahead with "a boxing match in which he boxes alone."
Rainsy said that the United States should focus on Cambodia after the wave of democratic reforms in Myanmar, where Obama has suspended most sanctions to reward changes.
Eni Faleomavaega, the top Democrat on the House subcommittee on East Asia whose views are sometimes contrarian, lashed out at attempts to curb aid.
Charging that US policy contributed to the rise of the Khmer Rouge, Faleomavaega called instead for greater US investment as well as debt relief for "a country that we failed so miserably."