Thought this was quite good on getting people to defect and not just because it makes an identical point to one I made around here the other day:

There’s no getting away from the fact that it is quite difficult to reconcile a “they’re all the same” argument with a “this person chose to leave one for the other” argument: if they’re all the same, why bother moving?

But the most important point is this one:

Someone who used to vote Tory not voting Tory now is one vote off the Tory column, and someone who used not to vote Labour voting Labour now is one vote on the Labour column, and some people can be in both categories at once, and the net impact of just one of these people on the difference between the total Tory vote and the total Labour vote is +2.

A bit wordy, but does very clearly make the point that Labour trying to win over Tory voters isn’t so much an indication of their political philosophy as an acknowledgement of arithmetic.