Yesterday, I said that Cameron and Osborne have a difficult job on their hands today. They need to tread the thin line between opposiing the measure in the opre-budget report on the grounds that they are fiscally irresponsible, whilst maintaining that tax cuts in principle are good but should not be paid for by deferred rises.
The problem they have is making that case in a simple narrative. The "tax bombshell" works, but if, as expected, the Chancellor outlines his means for clawing his big giveaway back, then the "they're being sneaky bastards and you'll have to pay for this mess in the future but they won't admit it" line just becomes "they're being bastards".
At this point Brown can try and push his "I'm doing something and you're proposing to do nothing". Ideally what Osborne and Cameron need to be able to do is stand-up, respond to the Chancellor's speech by ripping it apart where it fails, but then proposing what he should be doing instead.
The key here I think - and this is based on the leaked information thus far and not the secret squirrel surprises that will be in there - is to highlight that Brown is hitting the top end to fund the bottom end but completely ignoring the majority in the middle.
Remember that Brown has made much of his "success" at raising incomes. This is actually one of his weaknesses because a great number of people have now moved into the forgotten middle section and are by no means "rich". If Cameron and Osborne this group that I suspect will be screwed over by the Chancellor, then the message will chime farther.
Having said all this, who knows what little con trick we might see in the speech. A cut of a penny in income tax perhaps but a freezing of thresholds? Giving with one hand and clawing it back through inflationary pay increases perhaps?