Julia Gillard continues to give every appearance of clearing the decks for an early-ish election, and last night the Victorian Greens launched its Senate campaign — a reminder that the election will not just be about government, but will also elect a new half-Senate (no one seems to think a double dissolution is still a serious chance) to take office on July 1 next year.

So this is probably a good time to review some basics about how the Senate works. A half-Senate election chooses 40 senators, six from each state and two from each territory. They will join the six senators elected from each state at the previous election, to make a total of 76. (The territory senators don’t have fixed terms, so they will take office straight away.)

In the 20 years that this system has been running, almost every state at every election — 37 cases out of 42 — has split three-all between left and right (counting ALP, Greens, Democrats and NDP as left, and everyone else as right). Three of the five exceptions have happened at the past two elections: in 2004 the right won four seats in Victoria and Queensland, giving the Howard government a Senate majority, and in 2007 the left partially wound that back by winning four seats in Tasmania.

So the continuing senators this time around — the ones whose terms don’t expire until 2014 — give the Left a slight edge: 18 Labor and three Greens, against 16 Liberals, two Nationals and an independent (Nick Xenophon, who is basically a wild card but who I’m counting on the right).

That means that even if this year’s election returns to the default position of each state going three-all (and the territories one-all, as they always have), Labor plus the Greens will have a two-seat majority, 39-37, from July 2011. To prevent that, the Coalition and its allies would have to win four seats somewhere.

That’s not impossible — Western Australia looks like their best bet — but it’s a big ask: to win four seats you need four-sevenths of the vote, or about 57%. Queensland in 2004 is the only time the right have done it under their own steam, since in Victoria that year Steve Fielding only got up courtesy of preferences from the Democrats and ALP.

It’s at least equally likely this year that Tasmania again will elect four left senators (three ALP and a Green). There’s also a good chance that Victoria could join it (it would need a swing to the left of less than 1%), and some chance of the Greens winning a seat from the Liberals in the ACT (a swing of a bit over 1%).

It’s therefore much more likely than not that Labor and the Greens between them will have a majority in the new Senate, and since there’s no chance of Labor winning a majority in its own right, that’s another way of saying the Greens will have the balance of power.

But it’s appropriate that Tasmania’s Nick McKim was on hand for the Greens’ launch last night: just as in the Tasmanian election the Greens would have won the balance of power even if their vote had dropped (although in fact it increased), so they will hold the pivotal position in the Senate even if they lose seats, because what really matters is the balance between the major parties.