Thirteen Democratic lawmakers are encouraging the Department of Justice not to cave in the face of Republican opposition to a crackdown on online payday lenders.

In a letter Wednesday, Democrats including Sen. Jeff Merkley and Rep. Elijah Cummings told Attorney General Eric Holder that the Justice Department investigation known as "Operation Choke Point" should continue.

"Banks and third-party payment processors play a central role in the operation of the payment system," the lawmakers wrote, "and all Americans depend on the vigilance of banks and payment processors to ensure they do not become unwitting victims of fraudulent schemes."

According to the Justice Department, Operation Choke Point aims to prevent mass-market fraudsters, including illegal online lenders, from using the electronic payments system to access consumers' bank accounts.

But according to the investigation's critics, including GOP Rep. Darrell Issa, Operation Choke Point looks like an effort to stamp out even legal online lending.

As part of the probe, Justice Department investigators have sent more than 50 subpoenas to banks and third-party payment processors. They have also reached a $1.2 million settlement with the $809 million-asset Four Oaks Bank in North Carolina over alleged misconduct in the processing of payments for online lenders.

The Democrats' letter follows a request in early January by Issa and GOP Rep. Jim Jordan that the Justice Department turn over certain documents related to Operation Choke Point.

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