In what continues to be a nail-bitingly close contest, Democratic Party powerbrokers, plus voters in the remaining primary contests, have to choose between two very appealing candidates. In addition to their own views as to which would make the better president, they will also be looking at which of them has the better chance of beating John McCain in November.

Opinion polls consistently show Barack Obama doing better than Hillary Clinton when matched against McCain. But, just like Australia, an American election is not won on the popular vote. A party has to get votes in the right places: here, in the marginal seats; in the US, the marginal states.

That’s why Clinton yesterday invoked Ohio’s status as a “bellwether” state. Is she right? Do the results so far support the contention that she is better placed than Obama to win the key states?

To win the presidency, the Democrats need a uniform swing of 1.1% from the 2004 result. So I started by selecting as marginal the states with margins of 4% or less on the Democrat side of the pendulum, and 8% or less on the Republican side. (I use two-party figures throughout; Wikipedia has a good concise summary of the 2004 figures.)

Of those 23 states, Obama so far has won nine and Clinton eight, with six still to be decided (including Florida and Michigan, which are disputed).

Not all states are equal, but taking them by number of votes in the electoral college, which decides the presidency, gives an equally close result: Obama 83, Clinton 76. Obama’s wins are mostly medium-size states, while Clinton has won both large and small — they roughly cancel each other out.

If you go by delegates won, which reflects the size of the margin as well as the size of the state, it’s much the same: Obama 569 in the marginals to Clinton’s 528. (I’m relying on CNN’s delegate count.)

What about if we reduce it to just the very marginal? There are 12 states lying between a 2% Democrat margin and 4% Republican. Of those, Obama has won five, Clinton four, and three undecided.

Going by electoral college votes in those 12 puts Clinton in the lead, 34 to 27. But Obama is ahead on delegates won, 290 to 244 – his big wins in Colorado, Minnesota and Wisconsin making the difference.

So, as far as we can tell from the results to date, the marginal states are an accurate reflection of the picture as a whole. Obama is probably a little ahead, but there’s almost nothing between them.