WASHINGTON — A proposal to extend health coverage for Agent Orange exposure to Vietnam-era Navy veterans has the type of backing in Congress that normally would make supporters hopeful.

In the House, a bill granting the benefits has garnered a whopping 320 sponsors – almost 75 percent of all members have signed on in support. Nearly half of all senators also support extending benefits to the so-called “blue water” sailors who served aboard ships in ports and surrounding ocean during the Vietnam War.

“If you served just offshore, you don’t have presumed coverage,” said Rep. Chris Gibson, R-N.Y., a retired Army colonel who sponsored the House bill. “Members of Congress have to fight case by case … It should not have to be that way, they should get presumed coverage.”

But the legislation has collected dust for a year, failing to move past House and Senate veteran affairs committees that serve as a crucial first step on the road to making the benefits law. The Republican chairmen of these committees are skeptical of the science behind the exposure claims and concerned about the cost of new benefits. This has held up the proposals, frustrating supporters.

The window for Congress to act might be closing – despite the support — as lawmakers face the long summer recess, a fall schedule dominated by the presidential election and the end of the legislative session in December.

Gibson, Senate lawmakers and veterans groups, including Vietnam Veterans of America and Veterans of Foreign Wars, were set to rally on Capitol Hill on Wednesday in hopes of finally moving the bills ahead. The expansion of coverage has been sought by veterans for a decade.

“We’ve never been in a stronger positon to get it passed,” Gibson said.

Some veteran sailors contend dioxin-tainted herbicide runoff was sucked up through their ships’ water filtration systems and piped to crew, sometimes at concentrated levels.

Gibson said it is “very clear” that sailors were exposed and that their medical records show similar elevated risks for diseases such as cancer, diabetes and Parkinson’s disease as ground troops.

But the Department of Veterans Affairs in February reviewed its policy and decided it will continue to deny Agent Orange benefits to about 90,000 sailors who served aboard aircraft carriers, destroyers, cruisers and other Navy ships.

The VA does assume herbicide exposure and provide health coverage to the vast majority of Vietnam veterans who were deployed on the ground or in rivers and inland waterways during the war. But the agency found no basis to cover the sailors.

With the VA unwilling to change its policy, convincing the chairmen of the veterans committees to let the bills move forward could be key for supporters.

“We are trying our best,” said John Wells, the executive director of Military Veterans Advocacy, a Louisiana-based nonprofit group that is among six veteran organizations slated to rally Wednesday.

Wells said he is spending the week on Capitol Hill meeting with lawmakers and staff on the veteran affairs committees to advocate for the bills.

It will be an uphill battle.

Rep. Jeff Miller, R-Fla., said he believes the science is uncertain on whether the blue-water veterans should be eligible for Agent Orange benefits – a position shared by the VA following a recent independent study, according to committee staff.

In 2011, the Institute of Medicine examined whether sailors could have been exposed to herbicide but the results were inconclusive. Potable water systems in warships could have collected seawater polluted by land runoff and concentrated the dioxins in Agent Orange through distillation, the institute found.

“The committee was unable to state with certainty that blue water Navy personnel were or were not exposed to Agent Orange and its associated [dioxin],” the panel found, referring to a disease-causing contaminant in the herbicide.

Miller has asked the Defense Department to search for any residue in the ship filtration systems and records showing if the vessels were supplied with water from the Vietnamese mainland.

The findings could sway the debate over benefits in the future, staff said.

Meanwhile, the cost of expanding benefits is a sticking point on the Senate Committee of Veterans’ Affairs, which is chaired by Sen. Johnny Isakson, R-Ga.

It will cost about $90 million yearly to expand health coverage to the veterans, according to the Congressional Budget Office, and fiscal hawks in Congress require such new spending to be accompanied by cuts elsewhere.

“Chairman Isakson has consistently required all bills to be paid for before the committee can move on them, and S.681 has an estimated cost of $1 billion without any offsets,” the committee spokeswoman Lauren Gaydos wrote in an email response, referring to the estimated cost of the Senate version of the bill for 10 years.