The Australian Greens have a major problem.

Since April 2012, when leader Bob Brown announced his retirement, the party’s vote has crashed. According to Essential Research, the Greens vote was consistently 11% when Brown left. In May 2012, it dropped to 10% — not a big deal, given the volatility of a third-party vote. But in September it hit 9%, where it stayed until the end of 2012. There was a minor resurgence in February and March, then it went back to 9%. In May, 8s began appearing. It’s been stuck at 8% ever since, although last week it dipped to 7%.

These numbers are for general voting intention. The Greens’ vote will be higher in the Senate. But whether the party can achieve the same sort of results as it managed in 2010, and secure extra Senate spots in Victoria and NSW, and save its spots in South Australia and Western Australia, must now be doubtful.

This hasn’t been for want of attempts at product differentiation. Early in the year, leader Christine Milne rather theatrically announced she’d walked away from the Greens-Labor agreement, much to both sides’ relief. Whatever impact it had was temporary. Complaints about Milne’s presentational style began to circulate.

Whatever the case, Prime Minister Kevin Rudd’s announcement of the asylum seeker deal with Papua New Guinea is surely a gift for the Greens. If the Greens can’t slice off a small part of Labor’s progressive vote based on this decision, they will need to fundamentally re-evaluate their tactics. That was the basis for the party’s enormous success at the 2010 election. But this time, there’s no Bob Brown to welcome disgruntled major party voters.