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CNN  — 

A group of Senate Republicans Wednesday unexpectedly rejected efforts to advance a bipartisan bill that would direct the Veterans Affairs Department to carry out studies and clinical trials on the use of cannabis to treat veterans’ chronic pain and PTSD.

The vote on the procedural measure was 57-42, falling shy of the 60 need to move forward.

The move came after a “spirited debate” in the Senate Republican policy lunch, according to Sen. John Cornyn, a Republican from Texas, that took place just before the vote.

Critics warned of substantive, procedural and political concerns, including the recognition that if the bill passed the Senate, it could be perceived as a big win for Sen. Jon Tester, the Democrat who chairs the Veterans Affairs Committee and who is up for reelection in the red state of Montana.

Democratic leaders had expected the vote to be successful, in part, because it had unanimous bipartisan support in committee, and planned to complete debate next week and pass it.

Eight Republicans did support it, including Sen. Jerry Moran of Kansas, the top Republican on the committee.

Cornyn explained there are concerns about the methodology of the proposed clinical trials because “this retrospective study would be done strictly through volunteers who would come forward and talk about their experience with marijuana and PTSD,” and “it depends on people to self-select and we don’t know how that would skew the results.”

Cornyn said Republicans also didn’t have “assurances” they could offer amendments to the bill and said there are questions whether whatever passes the Senate will be taken up in the GOP-controlled House.

Cornyn suggested negotiations over the legislation will continue and the Senate may take it up back up soon. He described the vote Wednesday as “hitting the pause button.”

After the vote, Senate Majority Leader Chuck Schumer said it was “regrettable” the bill was blocked and said he hoped talks to revive it would be successful. In order to be procedurally positioned to call up another vote on the bill, Schumer switched his vote to “no.”