When Mitt Romney tries to avoid scrutiny for his exceptionally low effective tax rate by noting that, in absolute terms, he's paying "a lot" in taxes, he won't be fooling most of his political colleagues. It takes a special kind of affluence to reduce one's tax burden so dramatically. And despite their significant wealth most recent Presidential candidates have paid significantly more in taxes as a percentage of their incomes in the year (or two) before their campaign.
The exception is John Kerry. Though Kerry himself had a modest income (for a politician) his wife, Teresa Heinz, comes from great wealth and, like Romney, made millions in investment income in 2003 -- the year she and he both released their tax returns. Together, their effective tax rate was a bit lower than Mitt and Ann Romney paid in 2010.
Both Kerry and Romney are judged against the tax rates George W. Bush put into effect. Bush himself was paying Clinton-era rates, so even though he sold off the Texas Rangers and made a bunch of investment income before he jumped into national politics, he was only able to drag his effective rate down to 20.5 percent.
The other two didn't make out so well. John McCain and Barack Obama both ended up paying Buffett-rule-esque rates on their 2006 income, despite incomes that put both of them squarely in the top percent of earners.