Many congressional Republicans have long insisted that tougher enforcement should be the first step of any immigration law overhaul. But a report released Monday finds that the federal government already spends more on immigration enforcement every year than it does on all of its criminal law enforcement agencies combined.

The United States spent nearly $18 billion on immigration enforcement in fiscal 2012, about 24 percent more than it spent collectively on the FBI, the Drug Enforcement Administration, the Bureau of Alcohol, Tobacco, Firearms and Explosives, the Secret Service and all other criminal law enforcement agencies, according to the 182-page report by the Migration Policy Institute, a nonpartisan think tank.

“From the standpoint of resource allocations, case volumes and enforcement actions … immigration enforcement can be seen to rank as the federal government’s highest criminal law enforcement priority,” the study says.

In 1986, by contrast, when Congress passed the last sweeping overhaul of immigration laws, immigration enforcement amounted to less than 20 percent of the budget for criminal law enforcement agencies, said Doris Meissner, a co-author of the report and a former immigration commissioner under President Bill Clinton.

“This is a historic reversal,” Meissner said, adding that spending surged after the terrorist attacks of Sept. 11, 2001, which she said “embedded national security into every aspect of immigration enforcement.”

The study comes as Congress is preparing to debate a comprehensive immigration overhaul for the first time since 2007, when the last such effort collapsed.

President Barack Obama, Speaker John A. Boehner, R-Ohio, and Senate Majority Leader Harry Reid, D-Nev., have all said they intend to put immigration on the agenda in the 113th Congress. Interest groups representing a broad variety of constituencies have lined up behind the effort, and bipartisan working groups are meeting privately in both chambers to discuss areas of possible compromise.

The Migration Policy Institute report does not make policy recommendations to Congress ahead of the coming debate. Instead, it analyzes what Meissner characterized as the one major area of agreement between Democrats and Republicans on immigration policy over the past quarter-century: the need for more enforcement.

“There really has been one element of immigration policy that’s been a source of support across the board for a very long time, and that has been strengthened enforcement, especially border enforcement and deportations and removals,” Meissner said.

The study lists several notable examples of how explosively immigration enforcement has grown in recent decades. For instance, it notes, more than half of all criminal prosecutions brought in federal courts are now for immigration-related crimes.

In addition, it says, the two primary immigration enforcement agencies — Customs and Border Protection and Immigration and Customs Enforcement — refer more cases to U.S. attorneys for prosecution than all of the Justice Department’s law enforcement agencies combined.

One prominent conservative voice on immigration, Mark Krikorian of the Center for Immigration Studies, on Monday criticized the report’s methodology, saying that it lumped money spent on customs enforcement together with money spent on immigration enforcement without distinguishing between the two.

The nearly $18 billion cited by the report includes funding for “things such as screening cargo, checking the duty-free purchases you made abroad, and so on,” Krikorian wrote in a blog post. “It’s not so much for law enforcement in the G-man sense as it is management of the daily business of government, like the Post Office or highway maintenance.”

Although the Obama administration is seeking to create new pathways to legal residency for the nation’s estimated 11 million illegal immigrants, it has simultaneously kept up the trend of tougher enforcement.

Immigration and Customs Enforcement announced on Dec. 21 that it deported a record 410,000 illegal immigrants in fiscal 2012, up from 396,000 in fiscal 2011 and 392,000 in fiscal 2010.

Meissner said the federal government’s emphasis on enforcement may not be sustainable in an era of shrinking budgets. The report also notes that tougher enforcement — combined with a stagnant U.S. economy and greater opportunities for workers in Mexico — has slowed the influx of illegal immigrants from the southern border.

“The facts on the ground no longer support assertions of mounting illegal immigration and demands for building an ever-larger law enforcement bulwark to combat it,” the report finds.

Meissner added that Congress could curb the growing costs of immigration enforcement, should it agree to a comprehensive policy overhaul that includes a pathway to legal status for undocumented residents who might otherwise face deportation.

“If the Congress decides on a legal-status program of some kind, that removes an enormous number of people from the potentially removable population,” probably decreasing enforcement costs, she said.