United Russia very much relies on Putin's personal popularity. He isn't even part of their party officially but he lets them campaign with his likeness and everything. People are definitely more loyal to Putin than UR electorally, if Putin got a new party I think most Russians would follow him. If he died I think United Russia would really be left in the wind and probably torn apart by internal competition like what happened before Putin. Though I would expect that Putin would likely make preparations before his death, and designate a political heir for UR or hell maybe even kill the party apparatus before he goes and replace it with something else