Howard Gleckman issues a warning:

If Congressional gridlock sends the U.S. government tumbling over the fiscal cliff later this year, Americans could face an average tax hike of almost $3,500 in 2013. Nearly 9 of every 10 households would pay higher taxes. Every income group would see their taxes rise by at least 3.5 percent, but high-income households would suffer the biggest hit by far, according to a new Tax Policy Center analysis.

TPC found that if the tax hikes last the entire year—a big "if"–those in the top 0.1 percent would pay an average $633,000 more than if today’s tax rules were extended. However, even middle income households would take a hit: they’d pay an average of almost $2,000 more, and see their after-tax income fall by more than 4 percent. Such tax hikes would be "unprecedented," said the paper’s authors, Bob Williams, Eric Toder, Donald Marron, and Hang Nguyen.