Freddie, a thoughtful conservative blogger not previously known to me but linked by Andrew Sullivan, says the following about McCain's "socialism" attack:
"I think we could have an election that involves a major debate about the progressive income tax, but in order to have it, we'd have to have a candidate who is actually opposed to progressive taxation. The alternative to progressive taxation is a flat tax, and John McCain is not a flat tax supporter. If this 'spread the wealth around' argument is an argument with actual substance, instead of pure political opportunism, it has to be waged by people who are actually opposed to progressive taxation, in favor of a flat tax. John McCain, as much as he may want to limit the slope of the tax line, isn't in favor of a flat tax; it's not in his policy proposals at all. Could you have a simple 'let's have a more regressive tax scheme than we currently do' argument? Sure. But that can't be this scorched-earth, progressive taxation equals socialism argument the McCain campaign is making. It just doesn't make sense to have this extremist argument when the candidate making it isn't on one of the extremes."
Freddie has a point, but he doesn't go far enough. First of all, if you are truly anti-redistributive, a uniform head tax is as good a candidate as the flat tax, and perhaps better, for your preferred tax norm. Hard to choose between them on anti-redistributionist grounds when (absent a meaningful non-government baseline) we don't really have a way to measure the relationship between benefit and rising income.
But in any case the flat tax isn't actually flat. It has a zero bracket, which means that it has a progressively graduated marginal rate structure. Historical tidbit: when the Supreme Court struck down the income tax (pre-Sixteenth Amendment) in the infamous 1896 Pollock v. Farmer's Land case, it held that a flat tax with a huge zero bracket and one very low rate (maybe 2.5% or so?) was unduly progressive, violating equal protection and indeed being, in the Court's view, socialistic.
To believe in flat rate taxation as a matter of principle - leaving aside the formalism and incoherence of a principle that's based only on the revenue side of the budget without regard to the outlay side - you just cannot believe in a zero bracket. Otherwise, as the old joke goes, we have already established what you are (i.e., a "socialist" under the inane and intellectually bankrupt pretend view of the McCain campaign) and are merely haggling over the price.