However, things are starting to get messy with the election campaign starting to barge into the referendum debate.
Not only is Vote for Change piggybacking on Key's popularity, Key is bagging Winston Peters. In doing so, he is raising Peters' profile and - as Peters is the worst advertisement for MMP - bagging MMP in the process.
All this is giving Vote for Change a last-minute fillip. The anti-MMP lobby has struggled for traction. Efforts to push the cause of the supplementary member system as a suitable halfway house between MMP and first-past-the-post have been a flop.
While Key - with other National MPs - has occasionally murmured a preference for supplementary member, the politicians have been reluctant to engage on the referendum.
Supplementary member now appears doomed. It is most unlikely to secure anywhere near enough backing to emerge as the favoured alternative to MMP. That most likely means any run-off at the 2014 election would pit MMP against first-past-the-post, the system MMP replaced.
The pro-MMP lobby is trying to take advantage of this, reminding voters they will get an independent review of MMP if they vote to retain the status quo, with a look at prime voter gripes, such as the deal-making in Epsom.
Sidelights
Every which way: Pita Sharples insists the Maori Party opposes asset sales - but will support iwi getting first pick when shares in state-owned companies come onto the market.
They said it: "That's a good question. And I don't have a good answer." - Act leader Don Brash, asked by Kathryn Ryan on National Radio why he floated the decriminalisation of marijuana.
"I'll even shout him a cup of coffee. But it will have to be in a takeaway cup." - Winston Peters invites John Key to debate the issues at Peters' open-air public meeting in Auckland's Aotea Square tomorrow.