Cox didn’t serve as special prosecutor for long. On a Saturday night in October of that year, President Richard M. Nixon ordered Richardson to fire Cox — but Richardson refused, choosing instead to resign. Solicitor General Robert Bork stepped in and relieved Cox of his duties. It was, as the New York Times noted Tuesday, the last time that a president fired someone who was investigating him.

In the wake of Comey’s dismissal, some members of Congress — mostly Democrats — have called for an outside investigation of Trump and the possible links to Russia. Unfortunately for them, the power to appoint someone to do so lies mostly with Trump and entirely with his party — despite reforms that began after Nixon’s “Saturday Night Massacre.”

Before Nixon, special prosecutors were appointed by the executive branch — by the attorney general, usually at the behest of the president — on an ad hoc basis. As PBS reported in 1998, Nixon’s abuse of that authority prompted a call for an independent authority to urge investigations that were outside the control of any agency of government.

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The result was the 1978 Ethics in Government Act, signed by President Jimmy Carter, which obligated the attorney general to recommend the appointment of a special prosecutor whenever charges of misconduct were presented to his office. At that point, a three-judge panel from the U.S. Court of Appeals for the District of Columbia Circuit would determine whether a prosecutor was warranted.

After five years, the law expired, forcing Congress to review and renew it. Among the changes made in the first renewal was to give the attorney general more latitude in making a request to the court for a prosecutor and more latitude to remove someone from the position. Concerns about the constitutionality of the provision were settled by the Supreme Court in 1988, with Antonin Scalia casting the only vote in opposition to it.

Renewal of the legislation became a political football. When Bill Clinton took office, Republicans were frustrated by the use of the law to investigate Ronald Reagan and the Iran-contra scandal. Clinton backed renewal, and the law was continued — just in time for a slew of investigations of Clinton and his administration, including the most famous, Kenneth Starr’s investigation of Whitewater that eventually encompassed the Monica Lewinsky affair.

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In 1999, the law expired without renewal. As The Washington Post reported at the time, that meant that “responsibility for investigating the White House revert[ed] to the Justice Department.” As the law stands now, the attorney general — or, if the attorney general is recused from the case — an acting attorney general has the ability to appoint a special counsel when in the public interest. (The term “special prosecutor” was retired during that first renewal given the presumption of something to prosecute.)

There are several ways in which Congress could nonetheless spur an outside investigation.

The first is that Congress could create a select committee to investigate the ties, something that operates outside the jurisdiction of the existing congressional panels that are looking into the matter. This is what Congress did when investigating the handling of the 2012 attack in Benghazi, Libya. It’s important to note that the effect of such a committee is not the same as a special counsel or prosecutor, because Congress would need to agree on the scope and cost of such an effort, injecting partisan wrangling into the process.

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Congress could also pass legislation that would reintroduce the special counsel rules first authorized by Carter, allowing for an independent system to determine whether and when such an investigation should begin. The giant asterisk to this idea, of course, is that such legislation would have to be signed into law by Trump, which seems very unlikely.

The other possibility it that Congress could try to force Trump’s hand in appointing an independent investigator. This is how Cox ended up being appointed under the Nixon administration; Democrats, who controlled the Senate, threatened to delay Richardson’s nomination as attorney general unless he committed to appointing an investigator. Senate Democrats have already said they plan to slow the chamber down in an effort to pressure Trump.

The problem for those seeking an outside investigation, of course, is that Republicans retain control of Congress and the White House. Attorney General Jeff Sessions is highly unlikely to buck his boss and appoint special counsel (assuming that, as with Comey’s firing, he sets aside his prior commitment to recuse himself from questions about the Russia investigation). And congressional Republicans have shown little interest in disagreeing with Trump’s handling of Comey and even less interest in working with Democrats to give an outside party or group control over the Russia investigation.

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Trump is the beneficiary, then, of a number of factors: The expiration of a law that could have forced an outside investigation, his party’s control of both sides of Congress and, perhaps most important, a starkly partisan moment in American politics that predisposes members of his own party to stand beside him.

Polling from March indicates that a majority of Americans would like to see an independent investigation — but among Republicans and Trump supporters, it’s a minority. At the same time, those groups view Congress much more favorably than they used to, and they’re disinclined to think that there’s much to the Russia story. In other words, there’s little political incentive at this point for congressional Republicans to buck the president — a president who’s popular with Republicans.