PHNOM PENH, Cambodia — The president of Cambodia’s main opposition party was formally charged with treason on Tuesday, after being accused by the country’s authoritarian government of plotting to overthrow its leaders with the backing of the United States.

The charge comes amid a wider crackdown on dissent ahead of parliamentary elections next year, with a particular focus on groups linked to Washington. If convicted, the opposition leader, Kem Sokha, could be jailed for up to 30 years and his Cambodia National Rescue Party could be dissolved, under the terms of a law amended this year.

Mr. Kem Sokha’s daughter, a party spokeswoman, said the opposition was in severe crisis since her father’s sudden arrest on Sunday, when armed police officers raided his house in the middle of the night and drove him to a remote prison on the border with Vietnam.

The daughter, Kem Monovithya, said her father had been charged by a prosecutor who had traveled to the maximum-security prison, about 120 miles northeast of Phnom Penh.