WASHINGTON — After nearly two years of sending conflicting signals on the legalization of marijuana, the Obama administration finds itself under increased pressure from all sides to deliver a consistent message on where it stands.

Democratic senators from Washington state and Colorado entered the fray Tuesday, releasing a letter that complained that federal agencies “have taken different approaches that seem to be at odds with one another.”

The senators — Patty Murray and Maria Cantwell from Washington state and Mark Udall and Michael Bennet of Colorado — cited two decisions this year that have puzzled proponents and opponents alike.

In February the administration said it would advise U.S. attorneys not to prosecute banks that illegally allowed marijuana stores to open accounts and accept credit card payments. But in May, the Bureau of Reclamation said it wouldn’t allow any federally controlled water to be used on marijuana crops because Congress had banned the drug.

The White House is getting an earful on the subject this week after The New York Times called Sunday for ending the national prohibition of marijuana.

While the senators want the federal government to back their states in taxing and selling marijuana to users over 21, legalization opponents say President Barack Obama isn’t doing enough to enforce federal laws that prohibit the drug. They want him to convene a gathering of scientists and health care experts to put a spotlight on the issue.

“We can no longer accept a see-no-evil, hear-no-vil marijuana policy,” said Patrick Kennedy, a former Democratic congressman from Rhode Island who’s a co-founder of the anti-legalization group SAM (Smart Approaches to Marijuana). “Negative consequences are mounting.”

Washington state and Colorado opened pot stores this year after voters there legalized marijuana in 2012. Colorado went first, in January, followed by Washington state earlier this month. While the Justice Department said last August that it would allow the states to proceed, the White House said Monday that it remained opposed to national legalization. “The administration’s position on this issue has not changed,” White House press secretary Josh Earnest said Monday.

Earnest said Tuesday that he hadn’t seen the senators’ letter, but he noted that the Department of Justice has established “some guidelines for administering the law in the unique circumstances that exist in Colorado and Washington state.” He said he was unaware whether the administration was reviewing agency policies.