U.S. House Speaker Nancy Pelosi, a Democrat from California, speaks during a news conference on Capitol Hill in Washington, D.C., U.S., on Thursday, Feb. 7, 2019. Pelosi told reporters that House Democrats plan to mark up legislation that will address "the proliferation of guns' in the U.S. and was disappointed President Trump didn't mention the issue in his State of the Union address this week. Photographer: Al Drago/Bloomberg

Notwithstanding the Mueller ­investigation's conclusion that President Donald Trump did not conspire with Russia to win the 2016 election, many Democrats have not given up on impeachment. Advocates for impeachment point to the publicly available proof of Trump's misdeeds—the potential obstruction of justice flagged by Mueller, of course, as well as the evidence uncovered by the federal and state campaign finance and tax investigations in New York—and argue that the House must impeach Trump, even if the possibility of conviction in the Senate is remote.

But Speaker of the House Nancy Pelosi will not be deterred. She continues to maintain that, while investigations will continue, impeachment is off the table. This is a defensible position: contrary to the view of many impeachment proponents, the Constitution does not clearly mandate that the House begin impeachment proceedings. While Article I provides the House with "the sole power of impeachment" and Article II states that impeachment is warranted for "Treason, Bribery, or other high Crimes and Misdemeanors," the framers did not define "high crimes or misdemeanors"—or indicate whether impeachment should be mandatory when the House is faced with evidence of impeachable conduct.

The Constitution does vest the sole authority to initiate impeachment proceedings in the House, and Article II, Section IV does state that the president "shall be removed from office on impeachment for, and conviction of, treason, bribery, or other high crimes and misdemeanors." But this language hardly makes the initiation of impeachment proceedings by the House mandatory. What is mandatory is the president's ­removal "if" he is impeached and convicted of high crimes and misdemeanors.

Legal scholars and historians have explored evidence of what the framers might have intended in the provisions addressing impeachment, but actual precedent is scarce. The only two cases in which the House impeached the president can be characterized as partisan efforts by the party outside the White House. The least political effort in modern times involved the congressional investigation of President Richard Nixon, but he resigned before the House could issue articles of impeachment.

Fortunately, guidance on the question whether impeachment is mandatory can be found in the area of prosecutorial discretion. Prosecutorial discretion is simply the recognition that every prosecutorial authority, be it federal, state or local, exercises judgment in determining the individuals and entities to be prosecuted and the crimes to be charged. There is always more crime committed than can be fully prosecuted, and there are frequently legitimate reasons why those individuals and entities who are prosecuted are not charged with all the possible offenses they allegedly committed. No thoughtful person would contend that, just because prosecutors have the legal authority to pursue all crimes against all suspects, all crimes and suspects therefore must be prosecuted to the fullest extent. Rather, we expect prosecutors to exercise judgment in selecting both the people and the crimes to pursue.

Prosecutors balance a host of considerations in making these determinations, including the type of crime committed, the individual characteristics of the accused and the alleged victim, the broader community interests and concerns, the available resources, the sufficiency and quality of the evidence, the likelihood of a conviction, and even the political realities surrounding a potential prosecution. Reasonable disagreement with how a prosecutor exercised his or her discretion in a particular case should be distinguished from the claim that prosecutors must pursue prosecutions simply because they have the legal authority to do so.

Prosecutorial discretion gives us a lens through which to approach Speaker Pelosi's view on the impeachment of President Trump. Under the Constitution, the House of Representatives is in effect the prosecutorial authority where allegations of presidential high crimes and misdemeanors are concerned. And, like any other prosecutorial authority, the House has the discretion whether to pursue a prosecution in a particular case. In deciding how and whether to use this power, the House can—and should—take into consideration such factors as the seriousness of the allegations against the president, the sufficiency of the evidence, the time and available resources, the likelihood of an ultimate conviction, and the political realities.

In this case, it seems that Speaker Pelosi has made an assessment of these factors and concluded that this is not the time to initiate impeachment proceedings against President Trump, and that further investigation is warranted. Indeed, the Mueller probe was always only one piece of the effort to unearth evidence of Trump's alleged misconduct. Some might disagree with how the speaker is exercising prosecutorial discretion, but that does not mean that the House should not have discretion. As with ordinary prosecutorial decisions, moreover, the speaker's assessment is unreviewable—except, of course, through the political process itself, after a hearing in the court of public opinion.

Victor Hansen teaches criminal procedure and prosecutorial ethics and Lawrence Friedman teaches constitutional law at New England Law | Boston.