By declaring a national emergency Friday to build new sections of border wall, President Donald Trump hopes to unlock billions of dollars from a range of sources under the control of the executive branch. One major money pot he plans to rely on: proceeds from the controversial practice of civil asset forfeiture.

Of the $6.7 billion Trump hopes to pull from military construction projects and the Department of Defense, roughly 8 percent – some $600 million – will come from the Treasury Department's Forfeiture Fund , made up of proceeds from cash, cars and other property forfeited during investigations carried out by the Treasury Department and the Department of Homeland Security, which includes the Secret Service, Immigrations and Customs Enforcement, Border Patrol and the Coast Guard.

Civil asset forfeiture is intensely divisive, not only drawing consternation from Democrats on the left but also sowing divides between traditional law-and-order Republicans and small-government-minded Libertarians on the right. Under the practice, investigators can seize assets from anyone suspected of criminal activity – regardless whether they've been or ever will be charged with a crime.

The model has been around since Prohibition, but it was reinvigorated by a series of statutes in the 1970s and turbocharged during the war on drugs in the 1980s: Under a bill passed in 1984, police not only could seize the profits of drug kingpins and white-collar con men, but they could also begin sharing in the profits.

Law enforcement leaders and unions have hailed the process as a necessary tool, and it's yielded high-profile successes. Federal authorities, for example, used civil asset forfeiture to seize a sprawling ranch in Montana tied to Pablo Escobar, and the money shared by such seizures has helped build jails and buy new equipment.

But it's also unleashed a raft of abuses brought to light by news outlets and lawsuits, and which have brought intense scrutiny from Democrats and Republicans alike: small-town police departments that apparently target out-of-towners in so-called "cash-for-freedom" deals, arresting people passing through and demanding that they turnover their cash and car in exchange for not being charged with a crime; families whose homes were seized despite being charged with no crimes.

"Lots of forfeiture funds have been spent on training trips, officers' funds of a highly questionable nature," says John Malcolm, vice president of the Institute for Constitutional Government at the Heritage Foundation. "It's a skewed procedure. ... I don't want to do away with it, but I want to do civil-asset reform."

Photos: Trump and His Supporters View All 72 Images

Harnessing civil asset forfeiture wouldn't exactly amount to getting Mexican drug kingpin Joaquin "El Chapo" Guzmán Loera to pay for the wall, as Sen. Ted Cruz, R-Texas, proposed Thursday in an op-ed in The Washington Post. Though the former head of the Sinaloa cartel – convicted earlier this week in Brooklyn federal court – is believed to have made some $14 billion, the money, if ever recovered, would be seized through criminal, not civil, forfeiture, which comes with more stringent spending restrictions.

Much like the national emergency declaration itself, the legality of using civil asset forfeiture proceeds to fund the wall is far from clear. Trump's decision to declare an emergency marks the first time that a president has done so to free-up funds that Congress – the keeper of the country's purse strings – has explicitly declined to provide.

A chief concern is just how expansive emergency declarations can be: They activate a series of statutes that grant broad executive power and they open a range of potential funding sources, including the Treasury Department's Forfeiture Fund. Traditionally, such a declaration would appear to allow the president to tap into the fund, as long as the money was directed toward law enforcement purposes as required by statute.

"Certainly while it is in place the assets can and should be used productively," says Eugene Kontorovich, professor at the Antonin Scalia School of Law at George Mason University.

Whether building a border wall is a law enforcement purpose, however, is a matter of disagreement. Kontorovich contends it is "clearly law enforcement-related;" Malcolm, of the Heritage Foundation, agrees.

"Drug interdiction and human smuggling are clearly law enforcement activities here. You can get prosecuted for drug importation and human trafficking," Malcolm says.

Other legal experts are less certain. Ilya Shapiro, director of the Robert A. Levy Center for Constitutional Studies at the Cato Institute, a Libertarian think tank, contends that "this is a question of emergency statutory powers and constitutional separation of powers," one that "largely depends on how you define the relevant terms:"