SEOUL—South Korean President Park Geun-hye is widely expected to be impeached by lawmakers on Friday amid the country’s biggest political scandal in a generation, raising the prospect of early elections in Asia’s fourth-largest economy.

The 300-member National Assembly’s 171 opposition and independent lawmakers are all expected to vote for impeachment at a plenary session. Some political analysts said they expect them to be joined by at least 29 members of Ms. Park’s ruling party after its leaders said that they would allow members of their caucus to vote their conscience in the secret ballot. An impeachment vote would require the backing of 200 members to pass.

If the National Assembly votes to impeach Ms. Park, she would immediately hand over power to the prime minister, traditionally a ceremonial position, while South Korea’s Constitutional Court deliberates on whether to assent to Ms. Park’s impeachment.

“Even pro-Park lawmakers in the governing party​can’t defy the wishes of the people,” said Lee Nae-young, a political-science professor at Korea University in Seoul, who predicted impeachment. “They see that if the impeachment bill is voted down, the ruling party will suffer an even bigger political blow.”

Scott Seaman, a Washington-based analyst for Eurasia Group focused on South Korea and Japan, said that there is a 70% chance that Ms. Park is forced out of office, with most of the uncertainty coming from the outcome of the Constitutional Court.