At Forbes, Josh Barro explains why Glenn Hubbard is roughly right:

Romney economic advisor Glenn Hubbard has an op-ed in the Wall Street Journal, alleging that Barack Obama’s fiscal plan will necessarily lead to a tax increase on the middle class. Hubbard even puts a number on the required increase, saying that Obama’s spending plans will require an 11 percent across-the-board tax increase on people making less than $200,000 a year.
Secretary Treasury Tim Geithner responded harshly, telling CNBC’s John Harwood “That’s a completely made up, remarkably hackish observation for an economist.”
Sorry, Tim. Hubbard is right on the broad point—the President’s math doesn’t add up, and his budget plan leaves budget deficits that are too large in the out years, which will require further fiscal adjustments. If those adjustments happen on the tax side (i.e., if the president’s spending priorities get funded) then middle-class tax increases will likely be necessary. And though his method for reaching it is strange, I think Hubbard’s 11 percent figure is approximately correct.

Yet at the same time, Josh notes that the president is not alone in advancing budget proposals that implie a steep increase in taxes:

You could just as easily write an op-ed noting that Romney’s fiscal plans are unrealistic, and that the secret ingredient to making his numbers add up will have to be a broad-based tax increase that includes the middle class. When both parties’ presidential candidates put out budget plans that don’t work unless you raise taxes on the middle class, that should tell you something: that one of the fiscal changes that America needs in the medium term is a middle class tax increase.

There are, of course, better and worse ways to raise taxes, a point Josh has made on numerous occasions. But there is very little political incentive for the campaigns to outbid on each other on which middle class tax hike will be the most painless. Rather, both parties will insist that the middle class will remain untouched or that it will see tax cuts while either (a) no one will pay increased taxes or (b) only the top 2% of households or only 20,000 households will pay more.