With Pennsylvania going to the polls tonight, commentators have bemoaned the campaign’s concentration on trivia and scare tactics, and the lack of attention being paid to policy issues. But this being politics, in the end the thing that counts is the numbers, and a look at those suggests that Barack Obama is almost certain to end up as the Democratic nominee.

To win the nomination on the first ballot, a candidate needs 2,025 delegates. According to RealClearPolitics, Obama currently can count on 1,650, a lead of 142 over Hillary Clinton. (Other sources give similar but slightly different totals: the New York Times puts Obama’s lead at 157, AP says 141.)

Pennsylvania will elect 158 delegates; polls consistently show Clinton with a narrow lead, but Obama should pick up enough delegates to put him within about 300 of the required total.

After Pennsylvania, there are a further nine primaries to be held over the following six weeks, with a total of 408 delegates at stake – the largest bunch of them (115) from North Carolina, a state Obama is expected to win convincingly. So it would be surprising if Obama failed to win at least another 200 pledged delegates, leaving him just 100 short.

Then there are the superdelegates. Clinton has a narrow lead (258 to 234, according to RealClearPolitics) out of those that have so far committed themselves, but in recent months the movement has been almost all to Obama. With about another 300 still to declare, it would take an earthquake for Obama to not get well over the hundred he will need.

So Clinton’s goal in Pennsylvania is not to pick up x number of delegates, because delegate mathematics alone can’t win it for her. The goal is to win big enough to radically change perceptions and shake loose Obama’s hold on the superdelegates. A margin of less than 10%, which is all the polls are giving her, isn’t going to do that.

A big Clinton win would reinforce her message that she, but not Obama, can win the big states: of the other six largest states to vote, she has won all bar Illinois, Obama’s home state. But no-one really knows how important that will be come November; for what it’s worth, national polling still shows Obama doing better than Clinton when matched against John McCain.

Clinton’s claim that in the heat of battle she will stand up better to McCain ultimately rests on faith. It may be true, but if Democrat voters haven’t accepted it so far, it doesn’t seem likely that they will start now. And they would have to do so in unprecedented numbers for Clinton to have a chance.