a heartbeat away from being
heartbeat (away) from being (something)
In a position to move into a position immediately upon the absence of one's superior or predecessor (often assumed to mean from death). The position of vice president may not seem important, but whoever it is will be a heartbeat away from being president, so we should take that into account when voting.
a heartbeat away from being (something)
Cliché set to be the next ruler upon the final heartbeat of the current ruler. (The decisive heartbeat would be the current ruler's last heartbeat.) The vice president is just a heartbeat away from being president. The prince was only a heartbeat away from being king.