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.