What would happen if the president insists on making an appointment despite the fact that a key senator has invoked the rule of senatorial courtesy against the appointment?

If a key senator has invoked senatorial courtesy against an appointment, traditionally the president would respect this objection out of deference to the senator. The president may choose to nominate a different individual for the position to avoid conflict and uphold the practice of senatorial courtesy. Failure to do so could lead to opposition and conflict within the Senate, affecting the president's ability to get the appointment confirmed.