CHARLOTTE -- President Obama's campaign marked the first day of the Democratic convention today with another tax attack on Mitt Romney.

A new video again demands that Romney release more than two years of tax returns, and says the Republican candidate is using tax shelters and special loopholes to keep his tax bill low.

"We'll never know the full truth about Mitt Romney's investments and manipulation of tax strategies until he follows decades of precedent and releases multiple years of tax returns," said an Obama campaign statement.

The video tweaks Romney by replaying comments from his running mate Paul Ryan, who supports changes to "take away the tax shelters that are uniquely enjoyed by people in the top tax brackets so they can't shelter as much money from taxation."

The Republican candidate's taxes are expected to be a frequent target of criticism as the Democrats open their convention today in Charlotte.

Romney has released his 2010 tax return in full, and an estimate of his 2011 returns. He and aides have accused the Obama campaign of harping on taxes in order to divert voters' attention from the struggling economy.

"At this week's convention, the Obama campaign won't be able to talk about President Obama's record of chronic unemployment, falling incomes, and skyrocketing gas prices," said Romney spokeswoman Amanda Henneberg.

Last month, Romney also cited privacy issues, including his contributions to the Mormon Church.

"One of the downsides of releasing one's financial information is that this is now all public, but we had never intended our contributions to be known," Romney told Parade magazine. "It's a very personal thing between ourselves and our commitment to our God and to our church."