As fallout from special counsel Robert Mueller’s report continues to unwind, Democrats are divided about how to respond, with some progressive House members and at least three 2020 candidates calling for President Donald Trump’s impeachment and others adopting a wait-and-see approach.

House Speaker Nancy Pelosi acknowledged the divide Monday in a letter to the House Democratic Caucus, saying Congress should see a full unredacted version of the report that was released last week by Attorney General William Barr before rushing to start impeachment proceedings.

“While our views range from proceeding to investigate the findings of the Mueller report or proceeding directly to impeachment, we all firmly agree that we should proceed down a path of finding the truth,” Pelosi wrote.

However they choose to proceed, House Democrats — who are already conducting a number of investigations into Trump, including in the Senate and House Judiciary and Intelligence committees — will be operating under a different set of rules than Mueller used in his nearly two-year investigation into Russian interference in the 2016 presidential election.

In his report, Mueller said he followed Department of Justice guidelines that prohibit the indictment of a sitting president. Because of this, Trump would not be able to defend himself against any charges in court, Mueller wrote in the report. So the special counsel wrote that out of “fairness” he chose not to conclude whether Trump obstructed justice. (Mueller did not find evidence that Trump’s campaign conspired or coordinated with Russia on election interference).

MORE: Read the redacted Mueller report

Still, in investigating Trump for obstruction, Mueller’s team was bound by the basic guidelines of a criminal investigation. And in his final report the special counsel weighed his evidence using the same type of criteria that prosecutors are required to follow in trying cases before a judge and jury: whether a suspect is guilty beyond a reasonable doubt of committing a clearly defined crime, and whether they acted with corrupt intent — a key requirement for conviction in a court of law.

Congress is under no such obligations when investigating Trump.

The Constitution gives lawmakers much more leeway in investigating wrongdoing, including by the president. It’s a largely political process devoid of well-defined standards.

Before the report came out last week and in the days since, House Democrats have issued a flurry of subpoenas as they ramped up existing investigations into the president, including the spending by his inauguration committee, his tax returns, and the security clearance process for some White House officials.

Those investigations are likely to intensify now, as will calls for House Democrats to launch impeachment proceedings. Looking to the past can provide important clues about how Congress acts when taking on a president.

Lessons learned from Nixon and Clinton

The standards for congressional investigations and impeachment proceedings are confusing, because lawmakers invoke legal terms even when the processes are mostly political.

When it comes to impeachment, the Constitution grants federal lawmakers the authority to impeach a president for “treason, bribery, or other high crimes and misdemeanors,” leaving the definitions of those acts up to Congress on a case-by-case basis.

“There is no fixed standard for impeachment,” said Michael Conway, who was the legal counsel for the House Judiciary Committee during the impeachment inquiry into President Richard Nixon.

Most scholars agree the actual articles of impeachment — the documents that outline the charges against an elected official — are not legally required to include criminal offenses. Historically, though, these articles have included federal crimes.

The three articles of impeachment brought against Nixon in 1974, for example, included obstruction of justice, abuse of power and contempt of Congress. More than two decades later, the House voted on articles of impeachment against President Bill Clinton that included charges of perjury and obstruction of justice.

All of those charges are federal criminal offenses. But Congress can find a president or another official guilty of the accusations even if it is unclear whether they would be convicted in a court of law.

Congress accused Nixon of telling people close to him that he would give them “favoured treatment,” which was understood to mean a pardon, if they kept silent or offered false testimony.

Dangling a pardon is not a crime in itself, Conway said, making it a good example of the difference between congressional and criminal probes. Nixon’s nods at possible pardons may have been legal. But Congress was concerned with larger questions about democracy and the political system — not a narrow statute in the criminal code — as it weighed whether Nixon’s actions in office constituted obstruction of justice.

Similarly, in Clinton’s case, Congress specifically acknowledged the difference between its constitutional responsibilities to provide oversight of the executive branch, and the way federal prosecutors investigate potential illegal activity.

“Although, the actions of President Clinton do not have to rise to the level of violating the federal statute regarding obstruction of justice in order to justify impeachment, some if not all of his actions clearly do,” the House of Representatives said in its articles of impeachment against Clinton.

The articles outlined how he encouraged a witness to give false testimony and coordinated with Monica Lewinsky to fabricate a cover-up story about their relationship.

In the Mueller report, the special counsel found a number of cases in which Trump declined to rule out pardons for some of his former advisers, most notably his former campaign chairman Paul Manafort. The Mueller report also outlined instances in which Trump routinely praised witnesses who declined to cooperate with the government.

Whether Mueller considered or intended for Congress to make a decision about whether Trump committed obstruction is a matter of debate.

In the report, Mueller said he took the Justice Department rules into consideration and “determined not to apply an approach that could potentially result in a judgment that the President committed crimes.”

But he added that if Congress applied “the obstruction of justice laws to the President’s corrupt exercise of the powers of office” it would accord “with our constitutional system of checks and balances and the principle that no person is above the law.”

“That indicates a kind of handoff to Congress,” said Philip Bobbitt, a Columbia University law professor who worked in the Carter administration.

There is historic precedent for this kind of “handoff” from a special counsel. Watergate Special Prosecutor Leon Jaworski sent Congress a “Road Map” that was then used for impeachment proceedings. Decades after Watergate, the report was finally made public last year.

Barr has pushed back on the idea that Mueller left the decision up to Congress, saying in his press conference that criminal investigations are not conducted “for that purpose.” He said Mueller’s open-ended conclusion on obstruction left the final decision up to him as the attorney general and there was not enough evidence to charge Trump with obstruction.

Barr’s decision — heavily criticized because it came before the full report was available to the public — has no legal bearing on how Congress chooses to proceed.

As Democrats continue to call on the attorney general to release an unredacted version of the report, the party has several options for holding Trump accountable, including continuing with ongoing probes of the president’s businesses and conduct in office.

But if Congress does choose to interpret the findings of the Mueller report as impeachable offenses, this is how a proceeding would work.

If two-thirds of the House of Representatives votes to approve at least one article of impeachment, the president is impeached. Then, the process moves over the Senate, which holds a kind of trial. Members of the House act as prosecutors, the president typically has a defense team, and the Senate acts as the jury. If two-thirds of the Senate votes that the president is guilty, he is removed from office.

How public opinion plays a role

Lawmakers have only exercised their impeachment power against three presidents: President Andrew Johnson in 1868 (who was impeached by the House but acquitted by the Senate), Nixon in 1974 (though he resigned before the House could vote to impeach him), and Clinton in 1998 (like Johnson, he was also acquitted by the Senate, and two of the four articles of impeachment never made it out of the House).

“Unless there is a felony breach of the public trust, an impeachment article is not well-founded,” said Robert Ray, a former federal prosecutor who investigated the Whitewater scandal under Clinton.

Some Democrats are taking a cautious approach.

House Majority Leader Steny Hoyer, D-Md., told CNN after the redacted report was released Thursday that “going forward on impeachment is not worthwhile” with an election 18 months away.

Hoyer’s comments underscored the fact that launching an impeachment proceeding would be a big political risk for Democrats — something 2020 candidate and Vermont Sen. Bernie Sanders reiterated during a CNN town hall Monday night. Trump is unlikely to be removed from office by the Republican-held Senate, even if House Democrats impeached him. So far, only a few GOP senators, including Rob Portman of Ohio and Mitt Romney of Utah, have said publicly that Trump’s behavior in the Mueller report was troubling.

And Democrats could face a backlash in the 2020 election if voters disagree with how they handled the situation — something Republicans experienced in 1999 when their poll numbers dropped after pursuing an impeachment of Clinton that most Americans didn’t support.

Trump’s approval rating dropped 5 points to 39 percent after the Mueller report was released, according to a POLITICO/Morning Consult poll released Monday. But at the same time, only 34 percent of voters in that poll thought Congress should begin impeachment proceedings against the president.

Since the report’s release, Trump and his allies have gone on the offensive, and Democrats are already facing stiff resistance to getting a full, unredacted copy of the Mueller report.

The day after Barr released the report, House Judiciary Chairman Jerrold Nadler, D-N.Y., issued a subpoena for the full, unredacted version. The Department of Justice responded to the subpoena by saying it was “premature and unnecessary” because it is planning this week to send the ranking members of the House and Senate Judiciary Committees a copy of the report that has “minimal redactions.”

The redactions will likely include grand jury material, which remains secret by law. A ruling earlier this month from the D.C. Circuit Court of Appeals, which held that judges with few exceptions cannot order the release of grand jury material, affirms that the material is hard to get. By extension, the ruling could make impeachment proceedings more attractive because an earlier ruling allowed grand jury information to be given to Congress during the impeachment process.

“There are some interesting constitutional questions that are going to have to be resolved,” said Mark Tuohey, who was deputy counsel for Ken Starr during the investigation of Bill Clinton.

In addition to his subpoena for the full report, Nadler on Monday subpoenaed former White House Counsel Don McGahn to provide documents and testify about his work. Compelling McGahn — a former Trump official — to testify under oath about his conversations with the president about the Mueller investigation itself could be an important opportunity for building the Democrats’ case.

In her letter Monday to House Democrats, Pelosi warned about the danger of seeming too partisan and hinted at the complicated calculus of choosing their next steps.

“As we proceed to uncover the truth and present additional needed reforms to protect our democracy,” she said, “we must show the American people we are proceeding free from passion or prejudice, strictly on the presentation of fact.”