By Alex Bregman

Many Democrats on Capitol Hill are now demanding that a special prosecutor look into Russia’s meddling into the 2016 election in the wake of President Trump’s firing of FBI Director James Comey.

It’s even something Trump threatened Hillary Clinton with in a presidential debate. But what are special prosecutors, and why are they so special … and feared?

Broadly speaking, they’re lawyers appointed to conduct investigations on behalf of the federal government.

They’re often referred to as “special counsels” or “independent counsels” because they’re brought in from the outside, so that you don’t end up having the government investigating itself.

The drumbeat for one starts when some sort of scandal is brewing. It makes sense that the idea originated during what was perhaps the biggest scandal in modern American history: Watergate.

In 1973, members of the Senate threatened to hold up the nomination of then-Defense Secretary Elliot Richardson to be attorney general unless he assured them he would appoint a special prosecutor to look into the break-in at the Democratic National Committee’s Watergate offices in Washington, D.C.

His pick: Harvard law professor Archibald Cox. President Richard Nixon, however, was not a fan, and when Cox demanded that Nixon hand over the famous White House tapes, Nixon demanded that Cox be fired.

That clash ultimately led to what became known as the “Saturday Night Massacre” on Oct. 20, 1973, when both Richardson and his deputy resigned after refusing to fire Cox.

Nixon ultimately got his way, getting Solicitor General Robert Bork to fire Cox.

Facing impeachment over the Watergate matter, Nixon would resign less than a year later.

This all led Congress to put the role of a special prosecutor into law, in the Ethics in Government Act of 1978.

However, that law expired in 1999, leaving it up to the attorney general to appoint one when he or she wants to.

Even before Comey’s ouster, a majority of the American people said a special prosecutor should be appointed to look into Russia’s meddling in the U.S. election.

That, however, will be left to Attorney General Jeff Sessions’ deputy, Rod Rosenstein, because Sessions has recused himself from any investigations related to the Trump campaign.

If and when that happens, when it comes to the role of a special prosecutor, at least you can say, “Now I get it.”