Square’s mobile readers are headed to select T-Mobile retail locations Tuesday, and just in time for campaign season.

The Square mobile reader — a smartphone accessory with a card reader that you can plug into the headphone jack of most iPhones, iPads and Android devices — will be sold in T-Mobile stores starting Tuesday. The attachments, which are already sold in Apple stores and other retailers such as Target and Best Buy, make it possible to process credit cards via smartphone. That ability has become increasingly important for merchants who have historically run on a cash-only basis, such as food carts or farmer’s market vendors.

But where the mobile readers could really shine in the coming months is with campaign donations. Online payments are so 2008; in 2012, mobile payments are expected to be the disruptive fundraising force.

Politico reported Monday that the Obama campaign will be using the Square credit card reader; so is the Romney campaign, according to The New York Times. The Times reported that the Obama campaign has already developed a donation app that is compliant with the Federal Election Commission.

The Square devices won’t be available in all T-Mobile stores, but small business owners can get them from select stores in California, Connecticut, Florida, Massachusetts, Maine, North Carolina, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, Texas, Virginia and Washington state.

In the D.C. area, the Alexandria, Va.,store at King and Payne streets will carry the readers. Virginia stores in Richmond and Glen Allen will also carry Square’s accessory.

Square can be used with Apple’s iOS platform or with Google’s Android, and charges a 2.75 percent processing fee per swipe. T-Mobile has already been selling the Square readers in a bundle with its 4G smartphones for business, but is the first carrier to retail the devices in stores.

Verizon has a similar bundled deal for the rival Intuit card reader, as does AT&T.

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