When opposition leader Sam Rainsy last year turned his back on a promise to return to Cambodia and face a prison sentence dropped on him while he was overseas, he cited the potential for the government to use any subsequent protests or violence to derail election reforms.

“The ruling party wants to delay the election process, and to delay the election process, there must be incidents and escalating violence, which would give them the pretext they need to dismantle the CNRP, or to delay the election process,” Mr. Rainsy said at the time.

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He decided to flee to Paris rather than rocking the boat, banking on the idea that the CNRP would win the next national election in 2018 as long as the voter list was cleaned of the potential for voter fraud.

But there is more than one way to delay the election process. And while Mr. Rainsy’s decision not to return might have helped avoid an outbreak of political violence, the CNRP has found itself hamstrung by creeping delays in the implementation of electoral reforms it has pinned its hopes on.

The projected start of voter registration has been pushed back from March to May to July to August—and now perhaps September. Civil society groups this week expressed concerns about the National Election Committee’s repeated delays in its plans to build a clean voter list from scratch before the 2017 commune elections.

Sophal Ear, an associate professor at Occidental College in Los Angeles and the author of “Aid Dependence in Cambodia: How Foreign Assistance Undermines Democracy,” said it appeared that the CPP was purposely trying to slow down reform.

“When it’s this many delays, and the answers don’t add-up, you betcha. Drag your feet until the changes that would benefit your opponent are practically impossible, run down that clock!” Mr. Ear said in an email.

Until the NEC finally removes all the double and missing names from the voter list by re-registering all 10 million eligible voters, the opposition party will remain in perpetual fear of giving the CPP pretext to derail its coveted reforms, he said.

“The CNRP is in a poor position not just until the new voter list is completed, but until they actually are allowed to win,” Mr. Ear wrote. “I hate to quote Stalin, but he did say ‘Those who vote decide nothing. Those who count the votes decide everything.’”

Thanks to painstaking negotiations between the CPP and CNRP in the aftermath of the contested 2013 elections, those who count the votes now include members of both parties. However, the NEC’s chairman, Sik Bunhok, is a former CPP lawmaker, and its controversial secretary-general, CPP stalwart Tep Nytha, was recently brought back for another term of service.

And with the 2017 commune elections looming, Mr. Rainsy said in an email on Thursday that he believed Prime Minister Hun Sen was purposely trying to slow down the electoral reform process and derail voter registration “because he would definitely lose any free and fair election.”

CPP spokesman Sok Eysan, however, said his party was not concerned about the recent delays and added that it was silly for Mr. Rainsy to accuse Mr. Hun Sen of responsibility for the stalling.

“We are not worried because we just follow the law. For his side, the disease is to always worry. The NGOs and the opposition have the worrying disease,” he said. “The NEC determines this process—it is up to them.”

NEC spokesman Hang Puthea could not be reached. Khorn Keo Mono, director of the NEC’s communications department, said this week that officials were waiting on equipment and some bureaucratic processes before they could start the technical work of registration.

Kem Monovithya, the CNRP’s deputy public affairs director, said it was inevitable that even technical work would see political elements enter, given the stakes at play for both parties.

“The delay in voter registration concerns all of us. The whole electoral reform is a political process, more than just a technical one, that adds to the difficulty and speed of this reform,” she said in an email.

However, Ms. Monovithya denied that the CNRP had been hamstrung amid the delays.

“[The] CNRP is focusing on activities that precisely prepare us for upcoming elections, what we are doing now is exactly what a political party needs to do. We are not a movement, we are a political institution gearing up to take power in 2018,” she said.

But a clean voter list for the 2017 commune elections is key, with the results across the country’s 1,621 communes affecting the government’s composition into the next decade.

The elections will not only dictate who controls the communes—the most important level of government for regular interaction with the people—but the 11,450 commune councilors will be the only ones who vote in the 2018 election for the Senate, which has six-year terms.

Next year’s elected commune councilors will also be the voters in the 2019 elections for the district, municipal and provincial councils—fixing these managerial bodies in place for five-year terms.

Thun Saray, the head of rights group Adhoc, one of the 13 organizations that complained this week about the voter registration delays, said it was crucial to set a date for the commune elections and register the nation’s voters as soon as possible.

“If the NEC knows the date, they can accelerate the process and plan everything according to that date,” he said. “I don’t know if it is a technical problem or a political problem, but I believe they need to set a date.”

(Additional reporting by Kang Sothear)