library("RPostgreSQL")

drv <- dbDriver("PostgreSQL")
host = 'host.com'
dbname = 'dbname'
port = 1234
password = 'password'
username = 'user'
redshift_cert = paste0(FILE_PATH, 'redshift-ssl-ca-cert.pem')
pg_dsn = paste0(
  'dbname=', dbname, ' ',
  'sslrootcert=', redshift_cert, ' ',
  'sslmode=verify-full'
)

con <- dbConnect(drv, dbname=pg_dsn, host=host, port=port, password=password, user=username)

Error in postgresqlNewConnection(drv, ...) :
  RS-DBI driver: (could not connect user@host.com on dbname "dbname"
)

psql 'host=host.com dbname=dbname sslmode=require port=1234 user=user password=password'