Senators and veterans groups are working to convince a few last holdouts to stop blocking a quick floor vote on a bill to extend benefits for Vietnam veterans exposed to Agent Orange.

Advocates are lobbying President Donald Trump to sign the bill if the Senate clears it. But Republican Sen. Mike Lee of Utah has questions about whether science backs up the policy. And Budget Chairman Michael B. Enzi of Wyoming is concerned about its nearly $2.2 billion cost over a decade.

Lawmakers want to pass it as a stand-alone bill in the Senate through a unanimous consent request, which does not require an actual floor vote of all senators. An attempt to do so Monday, led by New York Democrat Kirsten Gillibrand, was unsuccessful after Enzi objected.

Given that the bill’s cost could require a waiver of pay-as-you-go budget rules, which require new spending to be offset by cuts or revenue elsewhere, there’s also a possibility that it could hitch a ride on any tax legislation that makes it through the lame-duck session.

House lawmakers passed the legislation under suspension of the rules by a vote of 382-0 in June.

The legislation would provide additional compensation to veterans potentially exposed to Agent Orange when serving off the coast of Vietnam during the Vietnam War. Benefits would come both in the form of disability compensation and health care. Veterans who served on the mainland were eligible for the increased benefits as part of the Agent Orange Act of 1991.

But in 2002, the Department of Veterans Affairs determined that service members must have actually set foot in Vietnam to receive disability status due to chemical exposure, a ruling that was also upheld in court in 2008.

That excluded a cadre of Navy veterans who served in submarines and on ships, for example, who might have been seeking new disability benefits when the VA made the 2002 change. The decision has precipitated a number of legal challenges, but so far, Vietnam vets based offshore have been excluded from the benefits.

ICYMI: What’s a Continuing Resolution? Decoding This Jargon As Congress Averts a Shutdown

Concerns of cost and evidence

Lawmakers and veterans organizations are pushing for compensation for these so-called Blue Water Navy veterans, pointing to what they call growing evidence that suggests that even people who never landed ashore in Vietnam suffered from exposure.

But some lawmakers question whether veterans who served offshore faced the same toxic exposures as those who served on land. Lee wants to wait for a VA report to assess whether the health outcomes that Blue Water veterans are experiencing are indeed due to Agent Orange exposure decades ago. That study is expected to be completed sometime next year.

Until that study comes out, Lee doesn’t think the Senate should advance the bill, his spokesman Conn Carroll said. “We think the science should inform the policy and not the other way around,” he said.

Yet such a study will likely be inconclusive, as over the years any number of risks — including age, cigarette smoking or other occupational hazards — could be responsible for the conditions some veterans now face.

Costs are also a concern. The Congressional Budget Office in late November essentially doubled its estimate of the bill’s price tag. Now, the legislation is projected to cost about $2.2 billion from 2019 to 2028, the CBO said. The cost went up because the budget office expects more veterans or their surviving spouses to be eligible for compensation than it originally predicted, the CBO said in a Nov. 29 letter to Enzi.

“On this bill, many of us have been recently made aware of the potential cost growth from the potential and the budgetary and the operational pressures that would happen at the VA,” the Wyoming Republican said on the floor Monday.

To offset the new spending, the House agreed to raise interest rates for home loans for active-duty service members. Currently, rates are higher for reserve members, and veterans groups say that having a single rate across the military makes sense.

But VA officials did not embrace the offset. At a September hearing, VA Secretary Robert Wilkie told the Senate Veterans’ Affairs Committee that the funding mechanism “puts a burden on young active-duty service members who are getting their first home and also puts a burden on disabled American veterans who live in higher-cost areas like Charlotte or Atlanta.”

Senate Veterans’ Affairs Chairman Johnny Isakson has remained bullish on the bill’s prospects.

“We’re going to be fine, and I think we’re meeting all the challenges, and I think we’ll be in good shape,” the Georgia Republican said last week.

Montana GOP Sen. Steve Daines, a co-sponsor, said he had several discussions with Lee over the issue and is still trying to convince him.

“This is an issue of justice. This is an issue of bureaucracy, frankly, not doing the intent of what Congress was, when it was originally passed. And this would clarify that so that we can make sure these vets get the benefits they deserve,” Daines said Thursday, when the Senate was leaving for the week.

Veterans groups were also hoping to make their case to Lee, emphasizing that they viewed the higher cost estimate as a worst-case scenario. Lawmakers believe that up to 90,000 veterans and their dependents could qualify for benefits under the bill, but the costs assume that none of them already get care from VA, which is unlikely.

If the bill stalls during this Congress, Senate Republicans could open themselves up to criticism. The bill’s supporters say it would easily get 90 votes if brought to the Senate floor, and they are frustrated that Majority Leader Mitch McConnell won’t just call it up.

“All too often members of this Congress are willing to pay lip service to the sacrifices our military and military veterans make, then fall into the trap of playing politics when there’s a chance to actually do something to help them,” Democratic Rep. Tim Ryan of Ohio said in a statement on Thursday ripping senators for stalling on the legislation.

Paul M. Krawzak and Niels Lesniewski contributed to this report.