Republican presidential nominee Donald Trump has been calling for a criminal investigation of his political opponent Hillary Clinton on the campaign trail this year. Trump is angry the FBI probe of Clinton's email server ended with no charges.

Now, he says, an independent outsider needs to look at the Clinton Foundation.

"The Justice Department is required to appoint an independent special prosecutor because it has proven itself to be really, sadly, a political arm of the White House," Trump said in Akron, Ohio, last week.

Trump has offered no evidence to back up his claims that the Justice Department has been corrupted. But his call for a special prosecutor sounds familiar to people who study the intersection of law and politics.

"If you look at the chronology, pretty much the political party that does not control the White House tends to want special prosecutors and independent counsel laws," said Ken Gormley, president of Duquesne University in Pittsburgh and the author of two books on special prosecutors. "As soon as the party is in the White House, they don't want it anymore."

In modern times, the idea first cropped up during the 1970s. President Richard Nixon notoriously fired a special prosecutor who was getting close to the tapes that would bring down his administration. Five months after what became known as the Saturday Night Massacre, Congress started work on an independent counsel law. The idea was to insulate sensitive criminal investigations from meddling by the White House.

The law would stand for about 20 years. It didn't produce many criminal convictions, but it did stir up a lot of trouble for presidents — and the people close to them. In 1996, Hillary Clinton became the first sitting first lady to testify before a grand jury. This one was investigating the Whitewater affair, an investigation that began with a real estate investment and morphed into scrutiny of President Bill Clinton's sexual improprieties.

Hillary Clinton was never charged with wrongdoing, but the investigation left a mark. Years later, Gormley interviewed Bill Clinton about it.

"He made clear that his decision to allow and endorse the appointment of an independent counsel in the Whitewater matter was one of the biggest mistakes of his presidency," Gormley said. "In fact, the words he used at one point is that it was like Wile E. Coyote and the Road Runner. And almost from the moment he was in the White House, there were political opponents using special prosecutors to bring him down, and they were off to the races."

By 1999, when Congress considered whether to renew the statute, opposition came from nearly all sides. Even Whitewater independent counsel Ken Starr told Congress the law had become a political weapon.

"The statutory mechanism intended to enhance confidence in law enforcement had the effect of weakening it," Starr said, in arguing the law should not be reauthorized. "Jurisdiction and authority over these sensitive matters ought to be returned to the Justice Department. And who will oversee them? The Congress, the press, the public."

That's exactly what happened. The law expired. But the Justice Department can still use special prosecutors. They're career lawyers, protected from political leaders at Justice by a firewall.

During the George W. Bush years, special prosecutor Patrick Fitzgerald charged and convicted a top aide to the vice president. Justice Department leaders went on to deploy special prosecutors to investigate the firings of U.S. attorneys and the destruction of videotapes that showed torture of detainees.

But authorities have relied on those prosecutors only sparingly, a position adopted by many, including Gormley.

"I think I've come to the conclusion that we are better off when we have less of these investigations rather than more," he said. "They should be reserved for very special and extreme occasions."

Otherwise, Gormley argued, they could distract from what's really important.

"As we as a country were just obsessed with the issue of Whitewater, Paula Jones, Monica Lewinsky and all of these distractions during the 1990s, there were people inside and outside the country literally plotting our attack," he said.

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