The 52-year-old victim received life-threatening injuries in a disturbance at a property in Stevenston, North Ayrshire, on Saturday.
Police were called out at around 3.15pm to Misk Knowes in the town, where they also found a 30-year-old man seriously injured.
Both men were taken to University Hospital Crosshouse.
The 30-year-old man was released after being treated, while the 52 year-old man was transferred to the Queen Elizabeth University Hospital, where medical staff described his condition as “critical”.
Detective Inspector Alan McAlpine said: “Inquiries are ongoing at this time to determine the circumstances surrounding this incident. Local residents may have heard a disturbance within the property or just outside at the time of the incident.
“This was a violent attack which has resulted in a man sustaining life-threatening injuries. We have conducted a number of door-to-door interviews in the area and continue to seek information from the public to trace the person responsible.
“At this time I would urge anyone who witnessed the attack take place, or who has any information that may assist the police investigation, to contact Saltcoats Police Office through 101.
“Alternatively, Crimestoppers can be contacted on 0800 555 111, where information can be given in complete confidence.”