Democratic Party leader Song Young-gil was attacked in Seoul by a man wearing a traditional robe who approached him from behind and struck him on the head with a small hammer, a video uploaded to YouTube by a Democrat campaigner showed.
Reuters could not independently verify the images, but party officials said Song was in a stable condition and a suspect was in police custody.
In a contest characterised by scandals, smear tactics and gaffes, the Democrats' Lee Jae-myung and Yoon Suk-yeol of the conservative main opposition People Power Party are running neck-and-neck.
Both men on Monday condemned the attack on Song, who is also Lee's campaign manager.
"Violence harms democracy, it can never be accepted," Lee told another rally in the southeastern port city of Busan, and wished Song a rapid recovery.
Yoon issued a statement saying any violence interfering in the election could not be justified, and hoping for Song's speedy recovery and all campaigners' safety.
President Moon Jae-in called the attack a "terror against democracy," his spokeswoman said.
The attack came after early balloting that ended on Saturday was blighted by long waits for coronavirus sufferers, while some voters received used ballots.
The two days of voting drew a record of nearly 37per cent of the electorate but also tarnished South Korea 35-year democratic history of tight and relatively transparent management of elections, and a mostly successful fight against COVID-19.
'Yoon's campaign said "an elementary classroom election could not be sloppier", while the country's National Election Commission (NEC) issued repeated apologies and hurriedly tightened its voting procedures.
As daily COVID-19 infections hover near unprecedented levels above 200,000 and more than 1 million receive treatment at home, parliament agreed to ease in-person voting by such patients.
Chaos erupted at many polling places during Saturday's special early voting for infected voters. Instead of letting the voters directly cast ballots, some election workers collected and carried them in a shopping bag or plastic bucket to place in ballot boxes, the NEC said.
Some voters received papers that had already been used, while others had to wait in long queues in the cold, with at least one reported to have fainted.
"All the problems resulted from our failure at thorough preparations, and we are fully responsible for falling short," the NEC said in a statement.
There was however no evidence of foul play, it added.
About 44 million South Koreans are eligible to vote for a successor to Moon, who is legally barred from re-election at a time of growing frustration over skyrocketing home prices, polarised politics and graft scandals.
