The presidential campaign of Sen. John McCain in recent days has used the image of Joe the Plumber to illustrate the negative impact that critics say Sen. Barack Obama’s tax proposals would have on small-business owners.

It is true that under the Obama plan, if Joe earns more than $ 250,000 in any year after 2010, his tax rates would increase to their pre-2001 levels. That could have some negative impact on Joe’s willingness to stay in business and hire employees.

With that fact established, the next step is to put the case of Joe the Plumber into context.

Is a plumber who makes more than $ 250,000 the typical taxpayer who would be hit by Obama’s proposed tax increase on upper-income Americans?

The latest available data from the IRS suggest the answer is no.

There are two categories in these statistics where small-business income can arise. The first is “business or profession” income earned by sole proprietors. This category of income shows up on 20 percent of +250K returns (returns with taxable income over $ 250,000) and represents 3 percent of that group’s income.

The second category is “partnership and subchapter S” income. (Subchapter S corporations can be owned by a single individual or a group of individuals. The income of these corporations is not subject to the corporate tax.) This category of income shows up on 35 percent of +250K returns and represents 16 percent of that group’s income.

The sum of these two categories gives us a total of, at most, 19 percent of +250K income from sole proprietorship, partnership, and subchapter S income.

Not all of this total comes from small businesses as most people typically conceive of them. At one end of the spectrum, much of this income is miscellaneous nonsalary, noninvestment income such as consulting fees or proceeds from sales on eBay — activities that have little to do with employing others.

At the other end of the spectrum, income from partners of large law and accounting firms is also included in this total. So a lot depends on the definition of small business you choose. But it seems safe to conclude that under any reasonable definition, less than one-fifth of +250K income is derived from small business.

The tactic used by the McCain campaign is a familiar one in the political battles for lowering taxes on the highest incomes. That tactic is to pick out the most sympathetic cases among the high-income taxpayers who would be subject to the increase and then focus on them as poster children in the campaign against the tax.

For example, in their battle to reduce estate taxes, conservatives focus on small family farms and ignore the fact that the vast bulk of the benefit of estate tax cuts accumulates to families who have more than $ 100 million and who may have never been on a farm.

And in the battle over capital gains taxes, conservatives focus on the millions of small investors who have some small stock holdings, but then ignore the fact that most of the benefit of capital gains cuts accumulates to a relative few of America’s most wealthy households.

Now the McCain campaign wants voters to focus on the hardship of Joe the Plumber and ignore the windfall for hedge fund managers, corporate CEOs, nonworking rich, and all the other fat cats.

No offense, Joe — you seem like a good guy — but you are being used. You are a front for the real winners under McCain’s proposed tax cut: our friends in the upper-income strata.