An attempt by U.S. Sen. Cory Gardner to allow marijuana businesses to obtain banking services has been thwarted by Gardner’s fellow Republicans.

The Colorado Republican had tried to add an amendment to a popular criminal-justice reform bill that would have allowed banks to provide services to pot retailers in states where marijuana has been legalized, even though it remains illegal under federal law.

Gardner on Monday introduced the measure, which was similar to a stand-alone law that he and Sen. Elizabeth Warren, D-Massachusetts, have been trying to pass for months without success.

> BACKGROUND: Gardner proposes granting marijuana businesses bank accounts (VIDEO)

On Tuesday he pitched the measure on the Senate floor. (WATCH his speech below.)

But 9News reports that Sen. Chuck Grassley, R-Iowa, chairman of the Senate Judiciary Committee, blocked the amendment.

“If there is an attempt to legalize [pot] across the country, we should have that debate and let Congress decide the issue instead of creating a back door to legalization,” Grassley said.

Grassley’s move came after Senate Majority Leader Mitch McConnell, R-Kentucky, fended off Gardner’s amendment to the crime bill and other proposed amendments via a procedural maneuver, The Denver Post reports.

Gardner later took to Twitter to say he will keep trying.

“Over 95% of the US population lives in a state where some form of marijuana is legal. This isn’t a fluke, the American people have spoken and this is happening. My amendment to the First Step Act has been blocked, but I will not give up this fight,” he tweeted.