Lautenberg has tried to make this argument before.
Behind the scary rhetoric lies a very disturbing truth: the watch list is compiled based on intelligence and suspicions. This proposal would establish a precedent for the government to take away 2nd Amendment rights without adjudication, without the right to confront accusers, without even knowing what the criteria are. This would give the Attorney General (and, by extension, the President) the power to put you on the list based on criteria of his choosing.
When you consider the ever-expanding list of what constitutes "domestic terrorism", it gets worrisome, doesn't it? Last month Bernard von NotHous in North Carolina was sentenced in federal court for creating silver "liberty dollars" as a barter currency. He faces up to 25 years for what US Attorney Ann Tompkins called "a unique form of domestic terrorism."
Tompkins went on to say, "While these forms of anti-government activities do not involve violence, they are every bit as insidious and represent a clear and present danger to the economic stability of this country." She added, "We are determined to meet these threats through infiltration, destruction and dismantling of organizations which seek to challenge the legitimacy of our democratic form of government."
So now Frank Lautenberg wants to give Tompkins' boss the ability to take away 2nd Amendment rights from people who fit that description....and to do it based on suspicion, association, and without due process.