Who is a journalist? That's not a question limited to college classes, like the one Steve teaches on media ethics at George Washington University.

Congress is now grappling with a definition as it debates a media shield law that would protect reporters against "unwarranted intrusion" by federal prosecutors, in the words of Sen. Chuck Schumer.

The bill passed the Senate Judiciary Committee with bipartisan support earlier this month and could soon be headed to the Senate floor. It sprang to life after a spate of intrusions by the Justice Department last spring graphically demonstrated how overzealous law enforcement can threaten the vitality of independent journalism.

In one case, the department subpoenaed the phone records of Associated Press personnel who had broken a story about a failed terrorist plot in Yemen. In another case, a Fox reporter was branded a possible "co-conspirator" in an espionage investigation, an unprecedented attempt to criminalize the act of newsgathering.

The ensuing outcry forced President Obama and Attorney General Eric Holder to revise Justice Department guidelines and make it harder for prosecutors to procure the testimony of journalists. But only Congress, they said, could decide who should qualify for those enhanced protections.

Defining a journalist is not easy, and the shooting at the Navy Yard here in Washington this week showed why. Amateurs rushed to help with the coverage, monitoring police scanners, taking photos and videos, tweeting and re-tweeting information.

Some of their contributions were very helpful, and crowdsourcing is a growing part of the media universe. Police pleaded for citizen journalists to help fill in the background of the Navy Yard shooter, Aaron Alexis.

But the amateurs also made a lot of mistakes, and one of them was to take information directly from police scanners and post it on social media as established fact. Mark Brady, an information officer for Prince George's County in suburban Washington, D.C., told AP that "People on Twitter take it for granted that (scanner chatter) is real and confirmed," when it's not. Relaying such data without confirming it is "asking for trouble," he said.

Jim Farley, an executive at all-news radio station WTOP, told the Washington Post that experienced journalists wouldn't commit that error. "We've always had a rule here: A scanner doesn't give you information; it tells you (whom) to call," he said. "It's not a source."

Yes, professional journalists make mistakes too. We've made plenty ourselves. But there is a difference between an amateur and a professional: a difference in training, standards, experience and purpose. And that difference should be recognized in federal law.

A shield law that applies to anyone with a laptop or cellphone would be meaningless. It could also be dangerous. One example: Every good journalist takes national security seriously, balancing the right of the public to be informed against its right to be safe.

The Washington Post did exactly that after receiving leaks about clandestine surveillance operations from Edward Snowden. It published the story but withheld the most sensitive details at the request of the Obama administration.

Should Wikileaks, which shows no such regard for security concerns, enjoy the same protections as the Post? What about Snowden himself? If he sits in the Moscow airport, writing a blog on his laptop, does that make him a journalist worthy of legal protection?

Obviously not. The line has to be drawn somewhere. And the Senate committee did a good job: It defines a journalist as someone who has had an "employment relationship" with a media outlet for at least one year out of the last 20, or three months out of the last five years. It also includes student journalists, as well as freelancers with a "substantial track record" of performance.

The lawmakers understand that the online landscape is changing rapidly, and not all journalists get regular paychecks or even freelance assignments. So they drafted a provision that allows a federal judge to include under the law anyone else who is engaged in "lawful and legitimate newsgathering activities."

A federal shield law is needed now more than ever. As Obama has demonstrated, politicians have a growing ability to communicate directly with citizens over multiple social media platforms, and avoid the questioning and scrutiny of independent journalists.

Republicans should realize they have a vested interest here. No White House, of either party, should have the power to intimidate its journalistic critics. So here's a way to get Republican votes for the shield law: Rename it the "Hold Obama's Feet to the Fire Act."

Steve and Cokie Roberts can be contacted by email at stevecokie@gmail.com.