When Congress returns from its summer recess, members will face a pivotal decision about the expiring Bush tax cuts. President Barack Obama has called for their permanent extension for singles with incomes below $200,000 and married couples with incomes below $250,000, but has proposed that most of the tax cuts for households with higher incomes be allowed to expire.

To buttress this position, the president and his supporters have repeatedly asserted that the expiration of these cuts will have little impact, because they affect only a tiny fraction of the wealthiest Americans, people who "can afford it."

Recently, for example, Vice President Joe Biden harshly rejected House Minority Leader John Boehner's assertion that the hikes would harm small...