Elizabeth May has credibility on the environment that goes beyond partisanship. If May endorses Dion for Prime Minister it has an objective flavour that makes her support relevant. May's isn't a Liberal, May worked for Mulroney, May takes the unprecedented step of sacrificing her party's own self-interest, in the name of what is best for Canada. That is a powerful statement, especially as it relates to a Liberal leader, desperate to look a credible alternative and get some traction. The May/MacKay battle will garner national attention, which gives May a audience to make her points, within the frame of indirect support for Dion. I view May's position as pure political gold for Dion.
Another advantage, May's move undercuts the NDP. I say this from the Liberal perspective, May's "decision" could make the NDP appear divisive. Here we have a Green leader, willing to work together, against a common foe. That reality sends a message to voters that we can't afford to divide the vote, with Harper the benefactor. If that sentiment takes shape, the soft NDP voter might be inclined to move to the Liberals. Layton could come off as partisan, the odd man out, as others move to work together. I really believe the NDP risks being boxed in, and any harsh attacks on the Liberals might fall flat. From the Liberal perspective, I see the Green move as indirectly hurting the NDP, which again says advantage Dion.
When Harper attacks the Liberal environmental plan as a economic killer that relies on taxes, Dion now has an ally to join the retort chorus. People tire of the Liberal/Conservative point, counter-point arguments, often time it's tuned out as naked partisanship. Enter May, the calm, respected voice, who has Dion's back and the dynamic changes. When Tories howl about inaction and 13 years, we now have an interesting perspective. Voters may ask themselves, if Dion is such a fraud, then why is the Green leader speaking highly, offering support. Please explain how Dion loses, or Harper gains.
I'm not sure that May's move is a good for the Green Party, it's risky, particularly with the growing support. However, if I focus on Liberal prospects solely, this alliance looks a winner, in many respects.