The Heck-Perlmutter letter is just one of several moves by lawmakers upset by the new policy. Sen Cory Gardner (R-Colo.) has gone so far as to threaten a hold on confirmation of Justice Department nominees based on the new policy.

After the Obama Justice Department released its memo, FinCEN adopted its own expectations for financial institutions dealing with marijuana-related businesses.

"In general, the decision to open, close or refuse any particular account or relationship should be made by each financial institution based on a number of factors specific to that institution," the 2014 policy stated. "These factors may include its particular business objectives, an evaluation of the risks associated with offering a particular product or service, and its capacity to manage those risks effectively. Thorough customer due diligence is a critical aspect of making this assessment."

Credit unions and their trade groups have said the decision to potentially crack down on marijuana-related businesses creates confusion for credit unions in states where it is legal.

An NCUA spokesman said the agency is reviewing the Sessions memo.

"In the meantime, the NCUA expects credit unions, as it has all along, to evaluate risk and have appropriate measures in place to mitigate that risk," he said, adding that the agency examines credit unions for compliance with Bank Secrecy Act and anti-money laundering rules.

Heck and Perlmutter asked FinCEN to make no changes to its guidance.

"We urge FinCEN to preserve this guidance to continue to support banking infrastructure and access to financial institutions for businesses that are operating in accordance with state and local law and abiding by eight other stated factors in your guidance," they said in the letter.

They said rescinding the guidance would inject uncertainty in the financial markets, adding that "attempts to disrupt this market are dangerous and imprudent."