get (someone) onside
To gain someone's support or approval. If we can get the CEO onside, it would definitely help us to further this initiative.
keep (someone) onside
To maintain someone's support or approval. If we want to expand our initiative, then we definitely need to keep the CEO onside.
get/keep somebody onˈside
get/keep somebody’s support: The government needs to keep the major national newspapers onside to help win votes in the election.