Mitt Romney's campaign, in its latest attempt to respond to calls to release more years of tax returns, announced Friday that the Republican candidate averaged a 20 percent effective tax rate for the previous 20 years and never paid less than 13.66 percent.

But the campaign is sticking by the candidate's vow not to release more than his last two tax returns, which critics have made into a potent campaign issue.

Romney also released his final tax return for 2011 on Friday. That document shows the Romneys paid $1.9 million in taxes on nearly $14 million in income, mostly from investments, giving them an effective tax rate of 14.1 percent.

The Romneys donated about $4 million to charity in 2011, or about 30 percent of their income, the document shows. Romney previously released his 2010 tax return but no others.

Democrats have called on Romney to release previous years' tax returns, as other candidates have in past elections, and some have suggested Romney is trying to cover up years when he may not have owed or paid any taxes.

But the campaign affirmed Friday in a blog post that "in each year" from 1990 to 2009, "the Romneys owed both state and federal income taxes," with the federal effective rate averaging 20.2 percent. Annual charity giving averaged 13.5 percent during that period, the campaign said.

The country's highest federal tax rate is 35 percent, though the Romneys and most Americans end up paying smaller percentages of their income in taxes after deductions and tax credits. Also, capital gains are taxed at only 15 percent.

President Obama and Michelle Obama had reported taxes of $162,000 on an income of $790,000 on their 2011 tax return, for an effective tax rate of 20.5 percent. Most of that income came from sales of the president's books.

The middle 20 percent of all American taxpayers has paid an average effective tax rate of 3 percent to 9 percent since 1990, according to the Tax Policy Center, while the top 1 percent's average rate ranged from 18 percent to 24 percent over that period.

The question of Romney's tax returns has been a thorn in the side of his campaign dating back to the Republican primary race, and it intensified when Democrats took up the fight to suggest the candidate had something to hide.

Most notably, Senate Majority Leader Harry Reid claimed to have spoken to someone involved in Romney's former venture capital firm Bain Capital who said Romney had paid no taxes for 10 years, though Reid has never named his source nor provided any evidence to back up the claim.

Romney has adamantly denied that claim saying at one point, "I have paid taxes every year, and a lot of taxes."

Reid and other Democrats have repeatedly invoked the example of Romney's father, George Romney, who as Michigan governor released 12 years of tax returns during his unsuccessful 1968 bid for the Republican presidential nomination.

Mitt Romney has countered that he already has done more than what is required. Presidential candidates typically release past tax returns, but they are not required to do so by law.