Hun Sen has ruled for 28 years, and his party has 90 of the 123 seats in the National Assembly. The prime minister recently said that he intends to stay in office until he is 74 cutting back from an earlier vow to stay in control until he's 90.
Critics of the government claim the election will be neither free nor fair, arguing that Hun Sen's regime manipulates the levers of government and influences the judiciary to weaken the opposition.
Last month, 28 opposition lawmakers were expelled from parliament when a committee run by Hun Sen's party ruled they had broken the law because they had originally won their seats in the name of the Sam Rainsy Party, but were campaigning under the recently established Cambodia National Rescue Party, into which it was merged.
They can still run in the upcoming election, but without parliamentary immunity. Immunity from arrest is a great benefit in Cambodia's elections, and those without it are at risk of being charged with defamation for remarks seen critical of Hun Sen and his government.
Rainsy is a charismatic and fiery speaker qualities that have landed him in trouble before.
He is expected to draw large crowds as he embarks on a whirlwind campaign tour that his party says will take him to over a dozen provinces in a week. He is likely to push hard on issues of corruption and land grabbing, with tens or hundreds of thousands of Cambodians displaced from their homes and farms under what are often shady circumstances.
Among the supporters at the airport was 74-year-old Chea Pirum who called Rainsy the politician he respected most in Cambodia.
"I've lived through five regimes and I have seen the other leaders, but Sam Rainsy is different," the man said. "He has devoted everything to the country, especially the poor, like me. I hope his return will bring full democracy."
Rainsy's pardon came after the U.S. and others had said his exclusion from the campaign would call into question the polls' legitimacy. Because he was absent during the registration periods, he will be unable to run as a candidate, or even vote, although his lawyers have said they were seeking a way to allow his participation.
"My return is no more than a step on a long journey towards achieving self-determination for Cambodia," Sam Rainsy wrote after he was pardoned. He criticized the official election body as unsupportive of democracy and said, "The mere fact of my return does not create a free and fair election for Cambodia."
The July 28 election will be the fifth parliamentary poll since the United Nations brokered a peace deal for Cambodia in 1991, a process meant to end decades of bloodshed that included the communist Khmer Rouge's catastrophic 1975-79 rule, during which an estimated 1.7 million people died in torture centers, labor camps or of starvation or disease.
