WASHINGTON — "Joe the Plumber" and other small-business owners want to know which presidential candidate would give them the best deal on taxes. It's a simple question with no simple answers.

The definition of "small business" is broad, ranging from two-person enterprises such as the one Samuel J. Wurzelbacher — "Joe the Plumber" — of Holland, Ohio, told Sen. Barack Obama he wants to buy, to a factory with fewer than 500 workers, to a construction company with $33.5 million or less in sales, according to the Small Business Administration.

Fewer than 2% of small-business owners take home more than $250,000 a year, according to the Tax Policy Center, a nonpartisan joint venture of the Urban Institute and Brookings Institution. Sen. Obama's plan raises marginal income tax rates only on individuals making $200,000 or more, and families making $250,000. Tax reductions can occur for people below those income levels via various tax credits. About 75% of small-business owners report business income as individual income, according to the Tax Policy Center.

Sen. Obama and Sen. John McCain have focused their discussions of tax proposals mainly on individuals and income thresholds. Sen. Obama's plan would raise the marginal tax rate on incomes above $250,000 a year to 36% and 39.6%, from the current 33% and 35%, effectively returning top tax rates to their levels during the 1990s.

"For the real Joe, if he makes less than $250,000, then he wouldn't see a tax increase. He'd get a targeted tax decrease," under Sen. Obama's plan, said Roberton Williams, principal research associate with the nonpartisan Tax Policy Center.

Mr. Wurzelbacher could also see lower taxes under an Obama administration if he qualifies for a variety of tax credits that the Illinois Senator has proposed, such as a $500 credit to match retirement contributions or an educational tax credit of up to $4,000 of college expenses.

But income taxes aren't the only issue for small-business owners.

Sen. McCain has proposed reducing corporate tax rates from 35% to 25%, but that would only potentially affect about a quarter of small-business owners, probably not including a business as small as Mr. Wurzelbacher's business, A.W. Newell Inc.

Health-care costs are also a consideration because of rising premiums. Only about half of small businesses provide coverage for employees, studies show.

Small-business groups have raised questions about Sen. Obama's health-care plan, which would mandate that some employers provide "meaningful" health-care coverage or give a percentage of payroll to fund a public plan. Sen. Obama has said small businesses would be exempt from that mandate. He has also proposed a 50% tax credit on employee health-care costs to allow more small businesses to cover workers, and the creation of a National Health Insurance Exchange for people to buy more affordable insurance.

However, small-business advocates say, they still have some major questions about Sen. Obama's plan. Namely, how he defines "meaningful" health-care coverage and how he defines "small business." Is it 10 employees or fewer? One hundred?

"Clearly, we don't believe mandating employers is the way to go about reform," says Amanda Austin, a health-care analyst at the National Federation of Independent Business. The group has remained neutral on the presidential race.

Sen. McCain has proposed ending the current tax treatment of employer-based health-care plans by making benefits taxable and offering offsetting tax credits. His plan would provide a refundable tax credit of up to $2,500 for individuals ($5,000 for families) to help purchase insurance.

"Both plans try to focus on affordability, but details are light," says Ms. Austin. "How do they get the costs down? If you get the costs down, you'll get more people coverage."

Sen. Obama would raise capital gains tax rates for families earning more than $250,000 to 20% from 15%. But the Obama plan would eliminate capital gains taxes for investors and entrepreneurs in small firms. Sen. Obama has proposed raising the cap on the 6% payroll tax used to fund the Medicare and Social Security systems, a move that would affect small-business proprietors. Sen. Obama has said he would consider levying Social Security taxes at a lower rate on income above $250,000. Currently, payroll taxes are applied only to the first $102,000 of annual income.

Write to Amy Schatz at Amy.Schatz@wsj.com