Even Libertarian Party candidate Bob Barr is going after the Joe Wurzelbacher vote:
ATLANTA, GA – “‘Joe the Plumber’ doesn’t need Senators Barack Obama or John McCain’” says Bob Barr, the Libertarian Party’s nominee for president. “He needs ‘Bob the Builder.’”
“While Senators Obama and McCain seek only to put government patches on today’s problems, ‘Joe the Plumber’ needs a leader who will rebuild our government into one that serves the interests of the people; not the interests of Washington,” says Barr. “Senator McCain wants to make ‘Joe the Plumber’ responsible for his neighbor’s mortgage, and Senator Obama wants to tax ‘Joe the Plumber’ for being successful. Both Senators Obama and McCain want to spend ‘Joe the Plumber’s’ tax dollars on wasteful projects, such as the $850 billion dollar bailout of Wall Street.”
“Senators Obama and McCain are no friend to ‘Joe the Plumber,’” says Barr. “But ‘Bob the Builder is.’ ‘Bob the Builder’ says that Joe should be able to keep the fruits of his labor, and shouldn’t be responsible for the mistakes of other people. ‘Bob the Builder’ says Joe shouldn’t be asked to shoulder Wall Street, and shouldn’t have his son sent off to die in a war that should have never been started. ‘Bob the Builder’ wants to build a government that is limited to its Constitutional authority, spends less of Joe’s money and stays out of Joe’s life.”
“America needs a builder, not a patch-job,” says Barr. “Senators Obama and McCain are a part of our problems—they are not the solution. America needs a leader with the vision and the courage to stand up against government, and truly reform it, where it once again begins serving the interest of every Joe and Jane across America. America needs Bob Barr.”
Joe the Plumber, meet Bob the Builder.

Forget the character attacks, however deserved or not. Let’s deal with the actual issue that was raised. Here’s a relevant excerpt from the (very) conservative Wall Street Journal’s article of OCTOBER 17, 2008, ‘As Joe the Plumber Grows Famous, the Politics Get Murkier’:
“To reach a level that would be affected by Sen. Obama’s proposed tax increase, Mr. Smither said, a mom-and-pop plumbing company like Newell would have to clear $5 million in annual sales. [Joe’s company actually reports only $100,000 in sales.]
Even if Mr. Wurzelbacher reaped taxable income from his business of $280,000 a
year, he’d pay only about $900 more a year in taxes under Sen. Obama’s plan, which
would raise the tax rate on the income between $250,000 and $280,000 to 36%
from 33%.
[...]
If Mr. Wurzelbacher earns the wages of a typical Ohio plumber, $40,600, and holds a $90,000 mortgage, he would see a TAX CUT (emphasis added) under Sen. Obama’s plan of more than $1,000, compared with no tax reduction under Sen. McCain’s.
If he succeeds in buying the plumbing business where he works, he could see even more tax benefits, including Sen. Obama’s proposed elimination of capital-gains taxes for small-business investment, a 50% tax credit to purchase health insurance for employees and a $3,000 tax credit for every new hire over the next two years.”
Yep … facts are pesky things.