The House and Senate conference on their annual defense policy bill isn’t happening. At least not today.

The House called off its vote on a motion to request a conference committee to hammer out differences with the Senate over the fiscal 2016 defense authorization bill (HR 1735), which was set to occur during Wednesday’s late-afternoon vote series along with a Democratic motion to instruct the conferees.

At issue is a constitutional requirement that bills affecting revenue originate in the House, and the parliamentary mechanism to enforce it, commonly known as a ‘blue slip.’

The chairman of the Senate Armed Services Committee, John McCain, R-Ariz., attributed the snag to his bill’s military retirement provisions, though did not specify further which part of the plan triggered the blue slip. Both House and Senate bills make changes to the retirement system for future service members by blending the current annuity benefit with a matching federal Thrift Savings Plan contribution.

Speaking to CQ Roll Call Wednesday afternoon, McCain expressed confidence he could get unanimous consent on the Senate floor to fix the issue.

“In other words, I have to get UC in order to take that part out of the bill, and I believe that I can get that tomorrow. I’m not sure, though,” McCain said.

House Armed Services ranking Democrat Adam Smith of Washington first told CQ Roll Call of the delay in the conference vote. Speaking off the House floor during votes, Smith said the issue was related to a blue slip against the Senate’s version of the bill, but stressed he knew little else.

“The Senate’s got a problem, so we’re not going to conference today,” said Smith, who would have led the minority side of the debate Wednesday as the top Democrat on Armed Services.

“I don’t understand the details. I was told there was a ‘blue slip’ revenue issue,” he said. “I am not a Senate parliamentarian. I don’t know what that means. All I know is it means that they have determined that the bill cannot go to conference and they have to fix it.”

The Origination Clause of the Constitution requires that revenue bills start in the House. As generally construed, the Senate may only attach revenue language to bills that have already passed the House that also contain revenue text.

The House may choose to send back legislation to the Senate without action with a blue slip of paper attached if it deems a bill to violate the Origination Clause. The easiest workaround for senators is to amend an unrelated, House-passed tax bill with the Senate’s legislation.

McCain said he was informed of the issue Wednesday afternoon and had spoken to House Ways and Means Chairman Paul D. Ryan, R-Wisc., whose committee makes blue slip determinations.

“He said he has no choice, but it’s funny that the [Senate] Finance Committee and others didn’t object, didn’t raise it,” McCain said.

The Senate averted such a blue slip snag with the fiscal 2013 defense law (PL 112-239) when it negated passage of its own bill, which included import sanctions against Iran and the Congolese rebel group M23, by unanimous consent and instead passed an amended House measure. The provisions would have affected tariffs and revenue, and were required to originate in the House. 

The House and Senate were off to an unusually speedy start with the policy bill — both chambers had passed the legislation before the end of June for the first time in years. McCain and his House counterpart, Texas Republican Mac Thornberry, have made bullish comments about their timeline for producing a final conference report to send to the White House, despite a promise from President Barack Obama to veto the legislation. Both first-year chairmen are aiming for final passage before the August recess, downplaying potential differences in their respective measures. 

A House aide brushed off the procedural snafu’s potential impact on the conference process. 

“Beginning and ending conference isn’t like flipping a light switch. There are always hurdles,” the aide told CQ Roll Call. “The two committees will continue reconciling the NDAA. By the time provisions get sorted, procedure should be sorted as well.”

Matt Fuller contributed to this report.