LINCHPIN

lynchpin [noun] [C] - an important member of a group or part of a system, which holds together the other members or parts or makes it possible for them to operate as intendedWoodford is the linchpin of the British athletics team.The king is the linchpin of national unity.The dollar is the linchpin of the system of international payments.(US) California was the linchpin (= most important) state in the last presidential election.