BANGKOK — Cambodians voting in Sunday's general election will have a nominal choice of 20 parties, but in reality, only two serious options: extend Prime Minister Hun Sen's 33 years in power, or do not vote at all.

The key factor virtually ensuring a walkover by Hun Sen's party is the elimination of any credible opposition, accomplished last November when the Supreme Court declared the Cambodian National Rescue Party complicit in trying to overthrow the government in a plot encouraged by the United States. The far-fetched allegation appears unsupported by any evidence.

The court ordered the party dissolved, also banning its leaders from holding office for five years and expelling its members from the elective positions they held. One party leader already was in exile and the other in jail awaiting trial on the treason charge.

Along with fracturing the political opposition, Hun Sen's government silenced critical voices in the media, shutting down about 30 radio stations and gutting two English-language newspapers that provided independent reporting. A law was passed putting burdensome restrictions on the country's vibrant civil society organizations.

On Saturday, Cambodian authorities acknowledged ordering the temporary blocking of 17 websites just ahead of the polls, citing regulations prohibiting media from disseminating information that might affect security after Friday's official end of campaigning. The blocked websites included those of U.S. Government-funded Voice of America, as well as local media.

Information Ministry Director General for Broadcasting and Media Phos Sovann said the websites, which he described as biased against the government, would be accessible again on Sunday.

With control of the legislature and the bureaucracy, as well as influence over the judiciary, there are no checks and balances on Hun Sen's administration.

"Cambodia's election is a sham process that is designed to prolong Hun Sen's authoritarian rule and will plunge the country into further misery and repression," said Debbie Stothard, secretary general of the Paris-based International Federation for Human Rights.

The leaders of the now defunct opposition party, most of whom have fled into exile to avoid arbitrary arrest, have called for a boycott of the polls.

"Going to vote on 29 July 2018 means that you play the dirty game of a group of traitors led by Hun Sen who is killing democracy and selling off our country," Sam Rainsy, the popular, self-exiled former leader of the Cambodian National Rescue Party, or CNRP, wrote on his Facebook page earlier this month. "Boycotting that fake and dangerous election means that we uphold our ideals by remaining loyal to our people and determined to rescue to our Motherland."

Ironically, a practice to fight voter fraud — dipping a finger in indelible ink to prevent multiple voting — makes Cambodians who fail to cast their ballots high-profile targets for any officials seeking to spot and punish opposition supporters.

The "Clean Finger" campaign promoted by the opposition is a form of political mobilization, said Mu Sochua, a former lawmaker and CNRP vice president.

According to her, not dipping one's finger in indelible ink is a political gesture: "This little finger that I have, that each of you have, is a symbol of what we stand for, what you want — democracy, freedom, liberty, justice."

Officials, claiming advocacy of the boycott is illegal, have made several arrests, but the opposition has effectively used social media to publicize its call.

Nineteen small parties have registered to challenge Hun Sen's Cambodian People's Party, but almost all are vanity affairs or vehicles serving as window-dressing to give the illusion of democratic choice.

Hun Sen has always ruled with a carefully modulated amount of repression, swinging between violence and reconciliation, but the slide into more serious authoritarian rule was triggered by the last general election in 2013, when the opposition CNRP won 55 seats in the National Assembly — a gain of 26 seats — while Hun Sen's party lost 22. The race was close enough for the opposition to claim that it would have won except for manipulation of the voter registration process.

In local elections last year, the CNRP showed a similar dramatic upward trend.

The results were alarming news for Hun Sen, who at 65 insists he will serve two more five-year terms.

Hun Sen can take credit for helping put an end to the long-running threat of the Khmer Rouge, the radical communist group whose 1975-79 genocidal rule left almost 2 million Cambodians dead. A Khmer Rouge officer himself, Hun Sen defected to neighboring Vietnam, with whose army he returned to help oust his former comrades. He became prime minister in a Hanoi-backed regime, and continued to battle Khmer Rouge guerrillas into the 1990s.

More recently, he has presided over a period of impressive economic growth that has helped fund the expansion of infrastructure, his major campaign promise.

But with economic growth came corruption, land-grabbing and cronyism as well as a culture of impunity typical of a broken justice system.

Demographics also appear to work against Hun Sen's party. A younger generation, without firsthand acquaintance of their country's history of war and instability, are less likely to pay heed to his warnings. Economic growth, as well as the expanded horizons that come with a connected, globalized world, fuel rising expectations.

The breadth and depth of Hun Sen's crackdown is a break with his historical behavior, where he teased, taunted, threatened and employed violence against his enemies, but usually paid at least lip service to the norms of democratic rule.

That seems all over now, said Sebastian Strangio, author of a 2014 biography of the prime minister.

Hun Sen was beholden to the Western aid donors who funded the massive peacekeeping and nation-building U.N. mission to rehabilitate Cambodia in 1992 and 1993. Introduction of a liberal democracy to replace the classical communist single-party state Hun Sen had been running was part of the deal.

Further financial assistance was needed to develop Cambodia, so Hun Sen at least maintained enough of a democratic framework to satisfy his benefactors. They tolerated a strongman who may not have been desirable but was capable.

If the 2013 and 2017 election results were the motivation for Hun Sen's smackdown of his opponents, China was the enabler, said Strangio, obviating the need for Western development aid by providing hundreds of millions of dollars of infrastructure loans and other forms of financing with few strings attached.

For its part, Beijing gets a solid political ally in Southeast Asia who can be relied upon in international forums to back up China's position on issues such as territorial disputes in the South China Sea.

"Over the past year, we've seen the government decisively roll back the zones of freedom it once preserved in Cambodia as a sop to Western donor governments," said Strangio.

"Now the government doesn't really have any need for Western support and they're able to make more permanent adjustments to the Cambodian political landscape in line with their long-held resentments and their current political interests," he said.