All attacks rebound in some way. Sometimes, they blow up in your face and other times, your shoes just get splashed a bit.
Today, Labour had a good time looking terribly 'in touch' in front of their supporters, and a bit smarmy in front of everyone else. The Tories look slightly off their game at the moment, but they still didn't need to work too hard in partly unpicking Ed's 'prolier than thou' credentials.
Still, it was bad day for the Tories and a not bad one for Labour, when all things are taken into account. It could have been a lot better though.
A union could have orchestrated a better stunt. A non-Labour affiliated one could have really gone to town on this one. Or any union's money could have hired the right celeb to take the piss out of Cameron all day long.
This has been in the Tory playbook for as long as anyone can remember. Deniable outriders make the attack before the front benchers turn up with a subtle and statesmanlike coup de grâce. A lot of the attacks Labour would like to make are out of bounds for two reasons: that they can rebound on the people who make them, and because they compromise future policy positions. Not a problem that the Tories have ever had with Taxpayer's Alliance attacks on Labour.
This is why party funding matters less than it seems. Labour and the big unions can help each other more if they start doing it informally.
This is an argument that's unlikely to find favour with Labour and senior trades unionists for all of the wrong reasons.