U.S. Sen. Elizabeth Warren is driving a push to make it easier for pot businesses to get access to banks and credit unions, calling the cash-dominated and nascent Bay State industry an “invitation” for criminals and fraud.

The Cambridge Democrat’s calls for greater clarity comes as Massachusetts has begun grappling with recreational marijuana, which officially became legal last month but stands nearly 18 months away from being sold in stores and dispensaries following a controversial late-session change by state lawmakers.

Warren, who helped pen a letter to the Financial Crimes Enforcement Network, said as the $7 billion marijuana industry has grown in some form in 29 states, but remains illegal on the federal level, the feds need to provide better guidelines to allow it, and the businesses it deals with, to get banking services.

“(In) these states, the majority of legal marijuana businesses, and businesses that provide services to them, are all but barred from participating in the financial system,” reads the letter, which also counts Vermont U.S. Sen. Bernie Sander as a co-signer. “As a result, many legal businesses are forced to operate in cash, which jeopardizes community safety, limits economic growth and greatly expands the opportunity for tax fraud.”

That type of environment, the lawmakers wrote, is “an invitation to tax fraud, robberies, money laundering and organized crime.”

Warren noted that since FinCEN last released guidelines in 2014, less than 3 percent of the nearly 12,000 federally regulated banks and credits union have chosen to serve the cannabis industry.

Warren said she was “open” to marijuana legalization before voters passed Question 4, ensuring Massachusetts joined Colorado, Washington and other states in approving weed for recreational use.

Her push for more clear regulations around the industry was applauded by medical marijuana patient advocates, who say added flexibility would trickle down to ease access for the public.

“This is going to put medical marijuana out in the forefront,” said Nichole Snow, executive director of the Massachusetts Patient Advocacy Alliance, which is slated to gather today to call on lawmakers to “tackle the problems” remaining with the medical marijuana law voters passed in 2012.

“If the businesses have more access to banking,” Snow added, “it would be a lot easier to apply to become a dispensary and then deliver medicine to the patients, who may not carry cash all the time.”