The brutal assault happened in the seaside village of Ardglass on Tuesday, May 22.
Detective Sergeant James Johnston said: “Police received a report that a 45 year old man had been seriously assaulted by a male in the Strangford Road area of Ardglass just before 11 am on Tuesday morning.
“It is believed the man was kicked to the head during the assault and as a result has sustained serious head injuries.
“He remains in intensive care in hospital this morning where his condition is described as critical.
“A 23 year old man was arrested in relation to the incident a short time later but has since been released on bail pending further police enquiries.
“We are currently following a definite line of enquiry in relation to this matter but I would appeal to anyone who was in the Strangford Road area on Tuesday and who witnessed the assault or has any information which could assist us with our enquiries, to contact detectives at Ardmore Station in Newry on 101 quoting reference number 377 22/05/18.
“Alternatively, information can also be provided to the independent charity Crimestoppers on 0800 555 111 which is 100% anonymous and gives people the power to speak up and stop crime.”