Given that between 1 and 2 percent of guns used by criminals were originally sold at gun shows and a similar amount in private sales. And given that many states do not report felony conviction data to the national database and even more do not report mental health information.
Given that federal gun prosecutions have dropped substantially, and that prosecutions of individuals who illegally attempt to buy a firearm and are stopped by NICs is vanishingly small.
Given that the only way to even theoretically make a universal background check work that includes person to person sales is to require registration. And given that even in Canada the long gun registration resulted in only 30 percent of long gun owners registering their guns. And that such a system in Canada cost billions of dollars, didn't solve one crime and took untold man hours of police time in addition to payroll.
Pursuing a Universal Background Check is a fools errand that will only succeed in making millions of Americans criminals, by definition of law, as polls estimate the overwhelming majority of Americans will not register their guns.
Maybe they could focus on more productive matters, like getting states to report felony convictions and those adjudicated mentally ill in a court of law, and funding mental health treatment to get people help so they don't reach the point of hurting others, and prosecuting federal gun crimes, and getting rid of gun free zones unless there is armed security provided.