As a depressingly exiled Democrat in the UK, I've been thinking that the attempted shutdown of London for three days is actually a potent campaign issue. So far, the Republicans have said that alienating allies doesn't matter so long as we're protecting ourselves, and they proceed to denigrate Europeans for being whatever they are.
But think about this: Bush needs to close down the capital of our closest ally in order to be safe during his visit. Clinton waded into crowds of thousands of adoring Vietnamese fans in the waning days of his presidency. Having to close down London is a powerful symbol of a foreign policy gone wrong. I think the Democrats need to play this up.