Congress has offered the public an unusual civics lesson: A piece of legislation can pass both houses of Congress and never reach the president’s desk – and veterans seeking easier access to medical marijuana aren't amused.

The House and Senate in quick succession passed a spending package Wednesday that will keep the government fully funded through Dec. 9., but lost in the hubbub was the fact that a popular amendment already passed by both chambers was missing.

When President Barack Obama signed the package on Thursday, it did not include a measure that would allow Veterans Health Administration doctors to fill out the forms necessary for veterans to acquire marijuana in states that allow its medical use.

In its final form, the spending package that passed Wednesday included the fiscal 2017 military construction and Veterans Affairs spending bill, to which the amendment had been attached before being abruptly stripped by a bicameral conference committee.

The Veterans Affairs' and military appropriations had passed the House and Senate in slightly different forms earlier this year as party leaders fought over the level of funding to address the Zika virus. But each chamber separately included the veterans marijuana amendment.

The marijuana amendment passed the House in May with a 233-189 vote. The Senate adopted a bill including the amendment in April after the reform was included by a committee following a 20-10 vote.

"The whole journey of [the amendment] has really defied mine and just about everyone else’s education about how legislation works in Congress," says Michael Liszewski, director of government affairs at the pro-medical marijuana advocacy group Americans for Safe Access.

"You’re brought up thinking it passes in the House and it passes in the Senate – great! – and then it goes to the president to get signed. But that didn’t happen here," he says. "It’s unfortunate and we may see this again, which is a shame because about 90 percent of Americans support medical access to cannabis."

Indeed, medical marijuana is overwhelmingly popular, with national polling showing support from about 9 in 10 Americans. Although cannabis remains federally illegal, half of states allow its medical use. Since 2014, Congress has voted to bar the Justice Department from undermining state medical marijuana laws.

A leading member of the conference committee that removed the amendment, Republican Sen. Mark Kirk of Illinois, infuriated pot advocates saying earlier this year "I don't think we have too few high veterans out there."

Though Kirk is suspected of playing an influential role in the amendment's removal, his office did not respond to a request for comment.

Rep. Earl Blumenauer, D-Ore., who spearheaded efforts to pass the amendment, tells U.S. News he intends to seek another vehicle for the measure so that veterans won't have to wait another year for lawmakers to consider it. He points out 57 Republicans joining all but five Democrats in favor in the House this year.

"We're watching a national marijuana movement build, especially for medical marijuana," he says. "The public wants it, Congress wants it, and to have that pulled out is to say the least disappointing, particularly when our veterans have been prescribed opioids at higher than the national average. But it's only a matter of time."

Blumenauer says he believes November votes on recreational and medical marijuana on state ballots could lend momentum to the push.

And pointing out that Kirk is in a tough re-election battle, he says "I think Sen. Kirk may end up not coming back because of his cavalier attitude."

Brandon Wyatt, an Iraq War veteran who uses medical marijuana in the nation’s capital, says he's "very, very disappointed” by the congressional action.

“Members of Congress have received kudos by feigning support of veterans healthcare," he says.

Wyatt, national policy adviser of the group Weed for Warriors, led a die-in near the White House in May, laying on a sidewalk with others and connecting widespread opioid addiction and the estimated 22 veteran suicides a day to the issue.

Research shows that states with medical marijuana laws have fewer opioid overdoses, and some veterans swear by marijuana as a treatment for post-traumatic stress disorder and depression.

Although half of states allow medical marijuana, cannabis remains a federally illegal Schedule I substance, meaning doctors cannot technically prescribe it but rather “recommend” it pursuant to state laws. The Schedule I designation means the plant has no recognized medical value and makes it more difficult for scientists to conduct research to prove or disprove its anecdotal utility for treating various illnesses.

The veterans marijuana amendment has had a tortuous time getting passed. It was approved by the Senate last year but was blocked by the House, where it failed on a 213-211 vote. One congressman told U.S. News he accidentally voted "no." Another said it didn't go far enough. Had they both voted "yes," it would have passed.

Wyatt says he's not as hopeful as Blumenauer about when the amendment may come up for another vote.

“I personally am not as optimistic because of the veil of secrecy they're using to do this," he says about congressional leaders.