Consumer advocates and financial-reform groups are lining up alongside some unions in opposition against the Senate's legislation that will impose regulations on the U.S. crypto markets, arguing that the current versions leave crypto consumers vulnerable to fraud.

"The legislative initiatives explored in the Senate so far have largely failed to address the

widespread harms caused by the crypto industry and the Senate should not consider any bill that does not address these problems in full," said the letter sent to senators this week, signed by almost 200 groups, including Better Markets, Public Citizen, Americans for Financial Reform and the Communications Workers of America. It followed a similar objection raised this week by the teachers' union, AFT.

Though the House of Representatives passed a bill earlier this year to govern crypto in the U.S., the Digital Asset Market Clarity Act, the Senate continues to hash out its own version, largely built on the Clarity Act but pursuing some different approaches. The senators negotiating the bill in closed-door session have said the process is nearing a conclusion, and one of them — Senator Cynthia Lummis — said on Tuesday that she hopes it can still get to a formal markup next week.

One of the major points of debate is the Democrats' concerns over apparent conflicts of interests in government officials — most notably President Donald Trump — being involved in crypto businesses as they're determining industry policy. The progressive groups' letter hit on that point, too.

"Any legislation must effectively address the unprecedented and corrosive impacts of President Trump’s and his family’s conflicted investments in various crypto ventures," it said.

Lummis said she'd worked on ethics provisions for the bill with a Democratic colleague, but the White House has so far rejected the proposals.

The latest opposition letter from consumer advocates was also signed by progressive environmental groups that don't usually weigh in in financial policy, including Greenpeace, the Center for Biological Diversity and Animals Are Sentient Beings, Inc.

A rift has developed among Democratic lawmakers on supporting crypto legislation, with Senator Elizabeth Warren, the ranking Democrat on the Senate Banking Committee, leading some of the more progressive members in criticism of the effort.

"This legislation poses profound risks to the pensions of working families and the overall stability of the economy," according to the teachers' union letter, which focuses its concern on the dangers to its members' retirement funds posed by the U.S. embrace of cryptocurrency. "Rather than just being silent on crypto, this bill strips the few safeguards that exist for crypto and erodes many protections for traditional securities. If passed, it will undercut the safety of many assets and cause problems across retirement investments."

Despite vocal pushback from some Democrats, Congress has managed to move forward with bipartisan votes on crypto legislation, including the new stablecoins law approved earlier this year.

Read More: Teacher’s Union Says U.S. Senate Crypto Bill Puts Pensions and Economy at Risk: CNBC