Last week, by a vote of 219 to 189, the House of Representatives approved an amendment aimed at stopping federal interference with state laws that "authorize the use, distribution, possession, or cultivation of medical marijuana." If it is included in the appropriations bill passed by the Senate and signed by the president, the amendment would prohibit the Justice Department, which includes the Drug Enforcement Administration (DEA), from spending taxpayers' money on dispensary raids or other attempts to stop medical use of marijuana in the 22 states that allow it.

Similar measures have failed in the House six times since 2003. This year the amendment attracted record support from Republicans, 49 of whom voted yes, compared to 28 last time around. "This measure passed because it received more support from Republicans than ever before," says Dan Riffle of the Marijuana Policy Project. "It is refreshing to see conservatives in Congress sticking to their conservative principles when it comes to marijuana policy. Republicans increasingly recognize that marijuana prohibition is a failed Big Government program that infringes on states' rights."

Yet Republicans still overwhelmingly opposed the amendment, by a ratio of more than 3 to 1, while Democrats overwhelmingly supported it, by a ratio of 10 to 1. Given the GOP's frequent lip service to federalism, the party's lack of enthusiasm for letting states set their own policies in this area requires some explanation. So does the need for this amendment under a Democratic administration that has repeatedly said it is not inclined to use Justice Department resources against medical marijuana users and providers who comply with state law. It is hard to say who is being more inconsistent: a president who promised tolerance but delivered a crackdown or members of Congress who portray themselves as defenders of the 10th Amendment but forsake federalism because they are offended by a plant.

During his first presidential campaign, Barack Obama repeatedly signaled that he would take a less repressive approach to medical marijuana than George W. Bush had. Campaigning in New Hampshire during the summer of 2007, Obama said raiding patients who use marijuana as a medicine "makes no sense" and is "really not a good use of Justice Department resources." In a March 2008 interview with southern Oregon's Mail Tribune, he went further, saying, "I'm not going to be using Justice Department resources to try to circumvent state laws on this issue." Two months later, when another Oregon paper, Willamette Week, asked Obama whether he would "stop the DEA's raids on Oregon medical marijuana growers," he replied, "I would, because I think our federal agents have better things to do."

Critics of the war on drugs were therefore puzzled that DEA raids on medical marijuana providers continued after Obama took office in 2009, even as the White House reaffirmed that "federal resources should not be used to circumvent state laws" and Attorney General Eric Holder claimed to be implementing that policy. "The policy is to go after those people who violate both federal and state law," Holder declaredduring a March 2009 session with reporters in Washington. "Given the limited resources that we have," he said during a visit to Albuquerque three months later, the Justice Department would focus on "large traffickers," not "organizations that are [distributing marijuana] in a way that is consistent with state law."

Deputy Attorney General David Ogden elaborated on that theme in an October 2009 memo, telling U.S. attorneys that "as a general matter" they "should not focus federal resources" on "individuals whose actions are in clear and unambiguous compliance with existing state laws providing for the medical use of marijuana." Ogden mentioned two specific classes of people who should be left alone: "individuals with cancer or other serious illnesses" and their caregivers. But he also listed criteria for federal prosecution, such as "sales to minors," "sale of other controlled substances," and "financial and marketing activities" inconsistent with state law, that make sense only when applied to suppliers. He warned that "claims of compliance with state or local law may mask operations inconsistent with the terms, conditions, or purposes of those laws"—meaning that federal prosecutors had to distinguish between bona fide medical marijuana dispensaries and fake ones.

Yet the DEA's raids continued. If anything, the pace picked up. Meanwhile, federal law enforcement officials such as Jeffrey Sweetin, the special agent in charge of the DEA's Denver office, and Melinda Haag, the U.S. attorney for the Northern District of California, publicly disavowed the notion that they needed to consider state law at all.

Pressed to explain the contradiction, the Justice Department issued another memo in June 2011. Backtracking in the guise of clarification, James Cole, Ogden's successor, insisted that prosecution threats against medical marijuana suppliers from Haag and other U.S. attorneys were "entirely consistent" with the Ogden memo, which he claimed applied only to patients and caregivers, meaning people "providing care to individuals with cancer or other serious illnesses, not commercial operations cultivating, selling or distributing marijuana." The Ogden memo's guidelines for distinguishing between genuine dispensaries and criminal fronts went down the memory hole, along with all of the assurances from Obama and Holder about respecting state law. In fact, since the Justice Department was now saying anyone but patients and caregivers was fair game for prosecution, Obama's policy was indistinguishable from Bush's.

Two years later came another DOJ memo and another reversal. This one was issued last August, after Obama was safely re-elected, after several more states had legalized medical marijuana, and after voters in Colorado and Washington, where cannabis was already allowed for medical use, decided to legalize it for recreational use as well. Still pretending merely to elaborate on what had always been a consistent policy, Cole said the Justice Department would not try to block legalization in Colorado and Washington as long as the new markets were properly regulated. He also suggested that it would not be a good use of DOJ resources to target marijuana businesses, whether medical or recreational, that comply with state law, provided they did not implicate any of eight "federal enforcement priorities," including prevention of interstate smuggling, violence, involvement by organized crime, and distribution to minors.

So far the Justice Department has indeed allowed legalization to proceed in Colorado, where state-licensed retailers have been selling marijuana since January, and in Washington, where growers have begun to produce marijuana for the recreational market and the first pot shops are supposed to open this summer. Yet the feds continue to go after medical marijuana growers and distributors, even when, as in the case of the Kettle Falls Five, they are clearly complying with state law. Hence the impetus for the amendment approved by the House last week, which aims to protect patients and providers from harassment, arrest, forfeiture, and prosecution.

My impression after following this story since 2007 is that Obama was sincere in saying state-legal patients and providers should be left alone but did not care enough about the issue to override resistance from the DEA and federal prosecutors (some of whom are more hostile to marijuana than others). Furthermore, he probably was surprised and embarrassed by the "Green Rush" that followed the 2009 Ogden memo, which encouraged many entrepreneurs to get into a business they otherwise would have considered too legally risky. But given growing public support for legalization (which got more votes in Colorado than Obama did in 2012), he decided it was best not to directly challenge that policy in states that choose to adopt it.

While Obama seems like a feckless and halfhearted supporter of marijuana federalism, most Republicans seem utterly unprincipled on this issue. Here is an opportunity to defend something they supposedly believe in—state autonomy under the Constitution—while simultaneously criticizing a Democratic administration and siding with a majority of Americans. In a recent Politico essay, Grover Norquist, president of Americans for Tax Reform, and Ethan Nadelmann, executive director of the Drug Policy Alliance, argue that opposing federal interference with medical marijuana is politically smart:

A recent Pew Research Center survey found that nearly three in four Americans—including 78 percent of Independents, 71 percent of Democrats and 67 percent of Republicans—believe that efforts to enforce marijuana laws cost more than they are worth. Similar numbers—80 percent of Democrats, 76 percent of Independents, and 61 percent of Republicans—favor making medical marijuana legally available.

The outlook for anti-pot Republicans looks even worse when you consider age trends. In a Gallup poll last fall, overall support for legalizing marijuana was 58 percent, including 67 percent of 18-to-29-year-olds and 62 percent of 30-to-49-year-olds. A CNN poll conducted in January put overall support for legalization at 55 percent and found a similar breakdown by age: Two-thirds of 18-to-34-year-olds said pot should be legal, and nearly as many 34-to-49-year-olds agreed. Are Republicans so blinded by anti-pot prejudice that they are willing to forsake political self-interest as well as principle?

Apparently yes. Not only have most Republicans failed to criticize the Obama administration's haphazardly heavy-handed approach to medical marijuana; many of them have faulted the administration for failing to block broader legalization in Colorado and Washington. They erroneously argue that using prosecutorial discretion to give states leeway on this issue violates the president's constitutional duty to uphold federal law. "Federal law takes precedence" over state law, Rep. Jason Smith (R-Mo.) told Holder during a congressional hearing last month. "The state of Colorado is undermining…federal law, correct? Why do you fail to enforce the laws of the land?" Not surprisingly, Smith voted against the medical marijuana amendment last week.

During the floor debate on the amendment, opponents questioned the medical benefits of marijuana and argued that allowing patients to use it is a prelude to broader legalization. Rep. Frank Wolf (R-Va.) cited opposition from professional medical organizations, while Rep. Andy Harris (R-Md.) argued that "it's the camel's nose under the tent." Yet neither of these arguments has anything to do with whether the federal government has the authority to stop states from allowing medical use of cannabis if that is what their legislators or voters decide to do. For a true federalist, this is the crucial question. Whether or not Wolf and Harris think medical marijuana laws are a good idea, our system of government does not allow them to impose that judgment on the states.

As reflected in last week's vote, there is a growing number of honorable exceptions to this overbearing, centralizing Republican tendency. Rep. Dana Rohrabacher (R-Calif.), who introduced the medical marijuana amendment, has been backing such legislation for more than a decade. This year he was joined by conservative Republicans such as Tom McClintock (Calif.), Paul Broun (Ga.), and Steve Stockman (Texas). Last year Rohrabacher introduced the Respect State Marijuana Laws Act, which goes further, declaring that the provisions of the Controlled Substances Act dealing with cannabis "shall not apply to any person acting in compliance with state laws." Republican cosponsors of that bill include Stockman, Justin Amash (Mich.), Don Young (Alaska), Thomas Massie (Ky.), and Duncan Hunter (Calif.).

Before Rohrabacher's amendment was approved, he urged his Republican colleagues to be true to their avowed principles. "For those of us who routinely talk about the 10th Amendment, which we do in conservative ranks, and respect for state laws, this amendment should be a no-brainer," he said. "Our amendment gives all of us an opportunity to show our constituents that we are truly constitutionalists and that we mean what we say when we talk about the importance of the 10th Amendment."

Norquist and Nadelmann likewise argued that it "ought to be an easy 'yes' vote for members of the 10th Amendment Task Force on Capitol Hill and other believers in limited government and federalism." The task force, founded in 2010, is a project of the Republican Study Committee (RSC), where conservative legislators are supposed to develop policies consistent with their principles. How many members of the RSC task force devoted to the 10th Amendment voted for federalism last week? According to a tally by the Drug Policy Alliance, 10 out of 47. That's 21 percent, up from 11 percent in 2012. It looks like Republicans may be gradually recovering from a cannabis-induced fog that made them forget the Constitution.

This article originally appeared at Forbes.