WASHINGTON — Hopes for a deal this year to extend disability benefits to thousands of “blue water” Navy veterans who claim toxic exposure while serving in the Vietnam War were all but ended Monday after supporters could not get unanimous support in the Senate for a potential fix.

The measure, passed by the House overwhelmingly earlier this year, would require Veterans Affairs officials to automatically assume about 90,000 veterans who served in ships off the coast of Vietnam were exposed to Agent Orange for benefits purposes.

That would move would bring that group in line with hundreds of thousands of other Vietnam veterans who served on the ground there or on ships traveling upon inland rivers. VA officials have objected to the plan, saying the available scientific evidence does not support extending the benefits.

Senate Veterans’ Affairs Committee Chairman Johnny Isakson, R-Ga., and other supporters had hoped to bring the issue directly to the chamber floor for a quick passage, but that parliamentary move was blocked by Sen. Mike Enzi, R-Wyo., who cited VA’s cost concerns about the measure.

“VA’s analysis shows the costs could be nearly five times what Congress assumed it was when the House of Representatives passed it,” he said. “So there's clearly more work to do just on figuring out the spending.”

Under current department rules, the blue water veterans can receive medical care for their illnesses through VA, but must prove toxic exposure while on duty to receive compensation for the ailments.

Advocates have argued that VA officials are systematically denying those claims despite cases of rare cancers and respiratory symptoms already linked to Agent Orange because not enough study was done decades ago to support the toxic exposure connections.

The veterans affected can’t afford more legislative delays.

“The only thing standing in the way of this bill to help our Vietnam veterans is the U.S. Senate, and that is shameful,” said. Sen. Kirsten Gillibrand, D-N.Y. “We have just days before this Congress is finished and our blue water Navy veterans are waiting for us. Their families are waiting for us. Some of them are dying waiting for us.”

Senate leaders could bring the bill up under regular order, but given the short schedule remaining — only a few days are left in the current congressional session — and their reluctance to make the issue a priority so far, that prospect is unlikely.

If the measure is not approved by the Senate by the end of the year, supporters will have to start over with the legislative effort again next year, a process that at best will likely take months.

Veterans groups called that a heartbreaking reality.

“If we can afford to send veterans to war, it’s unacceptable that we can’t afford to take care of them when they return home wounded, ill or injured,” said B.J. Lawrence, national commander of the Veterans of Foreign Wars. “(Enzi’s) obstruction forsakes our nation’s promise to take care of those who were injured or made ill due to their military service.”

Mike Little, executive director for the Sea Service Family Foundation and a longtime advocate on the blue water benefits issue, said senators should be “ashamed” they could not finalize a deal.

“Blue water Navy should be the easiest legislation passed by Congress,” he said. “The science is there, the facts are there, the advocacy is there. Nobody is lobbying against it except money hawks and the VA scientists who, on some levels, still disagree with the dangers of Agent Orange.”

The legislation carries a price tag of about $1.1 billion over 10 years, but VA officials have insisted the total is closer to $5.5 billion. The costs would be offset with a new fee on some VA home loans, a cost-savings measure some lawmakers have opposed.

Congressional supporters also expressed similar disappointment that a deal could not be reached. House Veterans’ Affairs Committee ranking member Tim Walz, D-Minn., blasted Enzi and several other silent opposers for being hypocritical about the costs, given the major tax package passed by the Republican-controlled Congress last year.

“It is especially troubling to see this bill blocked when there is so much work left to do on toxic exposures as a whole, to include those exposed to burn pits and radiologically exposed veterans,” he said.

His Senate counterpart, Montana Democrat Jon Tester, said the “this 11th-hour attempt to block the bill is a thumb in the eye to millions of veterans and service members,”

Isakson expressed frustration that existing research on the issue doesn’t seem to be enough for VA and some of his colleagues.

“This thing has been studied as long as it needs to be studied,” he said. “You’re putting off a decision we’re going to have to make in the future.”