Am not sure that any label can really work perfectly anyhow. I posed to an Australian Greens Senator in the pub the following:

If Greens ever become one of the two major parties then the ruthless power seekers will gravitate towards your party instead of the current major parties. How will the party be able to spot the wolves in sheep’s clothing?

His answer was “I don’t know. We discuss that often.”

Which struck me as the response of an honest and genuine person.