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As President Obama headed to Roseburg, Oregon, for closed-door visits with families of the victims of last week's shootings at Umpqua Community College, the White House is considering wading once more into the politically-fraught issue of gun control.

The administration is reconsidering a major gun control proposal that would establish new guidelines for who is legally defined as a licensed gun dealer and therefore required to conduct background checks on potential buyers, administration officials tell NBC News. The proposal would define anyone that sells more than a certain number of guns each year “in the business” of selling guns and therefore subject to laws for gun dealers.

According to sources, the administration has considered 50 or 100 guns a year as the threshold to trigger these requirements, although they have not formally settled on a number.

“This is a super-complicated policy,” said one administration official who was familiar with the idea.

In the wake of the deadly Oregon shooting in which nine people were killed and nine others were injured, the president pledged to have staff "scrub" existing gun laws in search of ways to push for reform. The president has often expressed frustration that political opposition has gotten in the way of gun control provisions.

Related: Obama Urges Public to Become 'Single Issue' Voters on Gun Control Reform

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Administration officials considered this executive order in 2013, after a bill requiring background checks for almost all gun sales failed to pass in the Senate. They ultimately opted against it, wary of both the complexity of defining who is a gun dealer and potential lawsuits arguing the executive order was illegal.

But they are reconsidering issuing such an order as part of a broader review of gun policy in the wake of the mass shooting at Umpqua Community College in Oregon.

The idea drew renewed attention when Hillary Clinton, a 2016 Democratic presidential candidate, said she would take such an executive action if elected. Everytown for Gun Safety, the Michael Bloomberg-funded gun control group, also called for Obama to adopt this policy.

Related: 'We Become Numb': Frustrated Obama Says American Gun Culture 'Doesn't Make Sense'

“These sellers are selling hundreds of thousands of guns without background checks,” said Adam Skaggs, senior counsel for Everytown.

The National Rifle Association blasted this idea.

There is little national data on gun purchases, so it’s not clear how many gun sales this would affect.

But gun control advocates say they are worried a growing number of weapons are sold outside the background check process, either online or at gun shows.

Some advocates of this executive order say it would actually expand the number of background checks more than the unsuccessful 2013 bill, which had a number of exemptions.

Obama aides have not given a timetable on when or if the president will determine if he wants to adopt this proposal. Obama has already used his executive authority repeatedly over the last two years to implement his policies, particularly on immigration.

However, Obama’s executive order to legalize millions of undocumented immigrants remains in a protracted court battle, and administration officials say this gun provision, if implemented, would likely be challenged by conservatives in courts.

But gun control advocates say Obama long ago should have issued the executive order and that he has bowed to political, not legal, considerations in not taking action. According to sources, some senior officials at both the White House and Department of Justice, including then-Attorney General Eric Holder, had argued the executive action was legal and proper when it was being considered.

Many of the key Senate races in 2014 were in pro-gun states like Alaska and Arkansas.

“I think there was a lot of pressure on the White House to let this go for a while,” said one gun control advocate, who is working with Obama administration officials on gun policy and did not want to criticize them publicly.

“Now, in his last 16 months, he can do this,” he added.