Not just Trump but also Mnuchin have been claiming that taxes won't decline for the high-income in the supposedly soon-emerging tax "reform" plan. The fact that Mnuchin is saying it, too, makes one think that maybe it isn't just impulsive lying or oblivious salesmanship, but reflects a view about what they will be able to claim about the ultimate package. Yet this is a deal that they are working on with Paul Ryan, not with Ron Wyden or Chuck Schumer.
So what might they be thinking, if we make the assumption (not 100% certain to be correct) that they actually are thinking strategically about the next stage? The following four things occur to me:
1) The fact that a corporate rate cut is likely to be at centerstage in the package may help them. Their argument that this doesn't help rich people can be both formalistic (corporations are separate taxpayers) and substantive (claims about the incidence of the corporate tax). Two points they will have to ignore, however, are (1) the transition incidence of cutting the corporate rate, which benefits existing shareholders if it wasn't fully anticipated, e.g., due to political uncertainty, and (2) the tax planning opportunities that a low corporate rate may give to high-income owner-employees.
2) They will be trying desperately to ignore the effects on high-income taxpayers of lowering the tax rate on amounts that business owners earn through pass-through entities. This is where the really big distributional action will be, if they go ahead with their reported plans to require people whom the tax system classifies as employees to pay significantly higher tax rates than those whom the tax system classifies as business owners. They'll call this a "small business" tax cut, but the point will be made in response that this a misportrayal of where the bulk of the benefit actually goes.
3) They anticipate announcing a package that takes away state and local tax deductions, and perhaps that also does something to home mortgage interest deductions. But these provisions seem highly unlikely to get through the Senate even if they are in the package and pass the House. Indeed, it's possible that they're being over-optimistic about the prospects even for unified "Big Six" endorsement of this.
4) Might they be planning not to propose repealing what is left of estate and gift taxes? Leaving that out of the package would certainly make it easier to claim that taxes aren't declining for the wealthy. But if I had to guess, they'll just try to brazen through the awkward tension between proposing estate and gift tax repeal and dressing up the package as no tax cut for the rich.