The 31-year-old man, identified only by his surname Kim, punched Rep. Kim Sung-tae on the chin in front of the National Assembly building in Seoul on Saturday.
The move by a Seoul prosecutors' office came hours after police requested the arrest of the suspect.
Police and prosecutors said they took into account "seriousness" of the case in seeking the warrant. They view the assault as premeditated.
"Kim has no fixed residence and there's a possibility of him running away," the prosecutors' office said, adding that prosecutors also considered the risk of Kim committing another similar crime, given his attitude after the attack against the lawmaker.
The lawmaker has been on a hunger strike since Thursday, calling for an independent counsel investigation into an online opinion rigging scandal involving a blogger with alleged ties to ruling party lawmaker Kim Kyung-soo.
The floor leader was treated and resumed his sit-in protest wearing a neck brace.
Police are looking into suspicions that he also planned to attack LKP chief Hong Jun-pyo, he added.
The assailant told police that earlier Saturday he went to Paju, north of Seoul, to protest against anti-North Korean activists' campaign of sending propaganda leaflets toward the North. After seeing the event foiled by police, he went to the National Assembly, where Kim was staging the fasting protest.
After the assault, the suspect yelled to express his hatred toward the conservative opposition party and call for the parliament to cooperate in promoting inter-Korean reconciliation.
"Let's achieve the reunification of the Korean Peninsula. Is it that difficult for the parliament to ratify the declaration?'" he was quoted as saying by LKP spokesman Chang Je-won.
In a historic inter-Korean summit last week, the leaders of the two Koreas signed the so-called Panmunjom Declaration to affirm their commitment to the complete denuclearization of the Korean Peninsula.
The National Assembly has been paralyzed over the past month as the parties have been at loggerheads over sensitive issues, including the online opinion rigging scandal, ahead of June local elections.
The violent attack led to the cancellation of a planned negotiation among floor leaders of the ruling and opposition parties, which was aimed at seeking a breakthrough for the stalled parliament. By the yonhap.