Each year, lawmakers quietly tuck language into spending bills that restricts the ability of the federal government to regulate the firearms industry and combat gun crime.

It’s the reason the Centers for Disease Control and Prevention can’t research gun violence, the Federal Bureau of Investigation can’t use data to detect firearms traffickers, and the Bureau of Alcohol, Tobacco, Firearms and Explosives can’t require background checks on older guns.

Since the late 1970s, more than a dozen provisions have been added to must-pass spending bills with no hearings, no debate and no vote.

Most of the recent talk about firearms has focused on a package of high-profile bills in the Senate. But as President Barack Obama readies his first budget since the Newtown, Conn., school shooting that left 26 dead, including 20 children, some advocates are urging him — largely behind the scenes — to delete the language that has been pushed by the powerful gun lobby in his spending plan to be released April 10.

“It’s not as well-known,” said Rep. Mike Quigley, D-Ill., who spearheaded an effort by members of Congress to send Obama a letter and introduced a bill to repeal some provisions. “But this is an inherent problem. It makes enforcement nearly impossible.”

Congress has approved stand-alone bills on firearms before, but as Capitol Hill becomes more acrimonious, lawmakers have attached measures to other bills. A provision allowing owners to bring guns into national parks, for example, was tacked onto a bill outlining restrictions for credit card companies.

But these days many policies that loosen gun laws appear in spending legislation, generally the annual appropriations bill for the commerce, justice and science agencies. Congress usually approves the bills with bipartisan support and the president signs the legislation into law. Once the so-called riders are in, they are difficult to get out, all but ensuring they remain in, year after year, no matter which party controls Congress and the White House.

Just last month, Congress made four of the temporary provisions permanent when it overwhelmingly approved a six-month funding bill to keep the government open. It was part of a deal struck last year between the Republican-run House of Representatives and Democratic-controlled Senate before the Newtown massacre, according to those familiar with the discussions.

The National Rifle Association, the nation’s most powerful gun rights organization, says the provisions merely correct oversteps in regulatory authority and that a slew of measures on other seemingly random issues also have been tacked onto spending bills.

“They’re trying to say we get a special deal with these riders,” NRA spokesman Andrew Arulanandam said. “It’s just the way business gets done in this town.”

In total, gun control activists say, the provisions limit how the government collects and shares information, oversees dealers, partners with state and local law enforcement, and researches gun-related deaths and injuries. Most policies pertain to the ATF by restricting its ability to manage data, make decisions and delegate functions to other agencies, such as the FBI.

For example, the agency can’t change the definition of antique firearms, which are not subject to federal firearms licensing procedures or background checks.

A series of those with the most impact passed in 2004, the so-called Tiahrt amendments, named after Todd Tiahrt, a Republican from Kansas who served in the House until 2011.

“I wanted to make sure I was fulfilling the needs of my friends who are firearms dealers,” he said at the time. Tiahrt, a defense and aviation consultant in Wichita, did not return a phone call seeking further comment.

The riders prohibit the ATF from requiring dealers to perform inventory checks, dictate that background check records be destroyed within 24 hours — making it difficult for police to notice who buys guns for others — and do not allow the government to create a centralized database of the millions of gun sales records they already have to determine where guns used in crime may have come from. Instead, the agency is forced to keep records in boxes in warehouses or on microfiche.

A provision that barred the CDC and the National Institutes of Health from spending money to “advocate or promote gun control” essentially halted research into firearm deaths and injuries. The language did not actually ban studies, but activists say the agencies believe they will lose funding if they conduct research.

The NRA’s Arulanandam said the policy was enacted because the federal government was not just researching but was promoting a political agenda.

For two decades, the federal government has conducted almost no scientific research on how criminals get guns, what triggers shooting rampages, why Americans use firearms more than other nations and whether more guns would bolster or hinder safety.

Major public research funding for gun prevention is $2 million annually — less than a hundredth of what is allocated to motor-vehicle safety, even though traffic accidents and firearms account for a similar number of American deaths each year.

Gun control advocates “have been outspent and outmaneuvered. They’ve lost on every front,” said William Vizzard, a criminal justice professor at California State University, Sacramento, who was a special agent in charge at the ATF. “They just want one victory.”