How to force a mongod to become primary in a replica set?

1) I have 3 mongodbs running in a replica set mongod --fork --logpath a.log --smallfiles --oplogSize 50 --port 27001 --dbpath data/z1 --replSet z mongod --fork --logpath b.log --smallfiles --oplogSize 50 --port 27002 --dbpath data/z2 --replSet z mongod --fork --logpath c.log --smallfiles --oplogSize 50 --port 27003 --dbpath data/z3 --replSet z 2) Now 27001 and 27002 are down. What do I need to do