The Obama campaign is asking supporters to sign a letter to Republican presidential candidate Mitt Romney asking him to release “just five years of tax returns.”

In an email to the list, campaign manager Jim Messina noted the letter he sent Friday morning to Romney’s campaign manager, vowing to get off Romney’s back about the returns if the documents were produced. The Romney camp brushed off Messina’s move.

Now Messina has converted his letter into an online petition of sorts.

“According to the one full year of returns he has released, Romney paid 13.9% in taxes on his income. Thursday, he said he went back and looked, and has never paid less than 13% over the last ten years. Now we’re asking him to put his money where his mouth is,” Messina wrote.

“It is absolutely relevant for us to ask how much a presidential candidate paid in taxes, if he sheltered his money or tried to get out of paying taxes at all, why he started — and continues to own — a corporation offshore in Bermuda, why he keeps his finances offshore in the Cayman Islands, and why he opened a Swiss bank account,” he continued.

In addition to being invited to sign the petition, supporters were reminded in a P.S. to open their wallets: “Election Day is closer than you think — make a donation to fund this campaign in the time we have left.”

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