The captain is PIC but must use his crew to keep him informed of problems or concerns they have with the operation of the flight. After their input it is his responsibility to do what ever is necessary to insure the safety and proper operation of the flight.
When he has made this decision and you as a crewmember choose to not follow his direction you are putting your job on the line and you know this.
I have done this a couple of times as an FO and both times knew the consequences but felt it was my duty. Both times the captain let it go. One time he thanked me. Do your job. Just make sure you are right if you push it.