For weeks, I have dismissed talk about the possible impeachment of President Donald Trump as premature and a waste of time, but I have changed my mind in light of evidence that he might have engaged in the crime of obstruction of justice. This is the crime that led to Richard Nixon’s resignation and Trump’s conduct is remarkably similar to that which caused the House Judiciary Committee to vote articles of impeachment against Nixon.

The Constitution provides that the president may be impeached for treason, bribery or “high crimes and misdemeanors.” Impeachment in the House of Representatives requires a majority vote, while conviction in the Senate requires a two-thirds vote. The phrase “high crimes and misdemeanors” never has been defined with any precision. Criminal activity always has been regarded as sufficient to meet this standard. But it also has been understood to include serious abuses of power, even if not illegal.

Only twice in history have presidents been impeached by the House of Representatives; in neither instance did the Senate vote to remove the president from office.

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Nation must remain patient as Russia probe plays out President Andrew Johnson was impeached for firing a Cabinet official in violation of a federal law, the Tenure in Office Act. After the assassination of President Abraham Lincoln, Johnson, a Southerner from Tennessee, succeeded to the presidency and found himself presiding over Reconstruction which he opposed. Congress passed a law, which today would be regarded as clearly unconstitutional, that deemed it a “high misdemeanor” for the president to remove any Cabinet official. After Johnson fired the Secretary of War, the House quickly voted articles of impeachment.

President Bill Clinton was impeached for lying under oath about whether he had a sexual relationship with a White House intern, Monica Lewinsky.

The House Judiciary Committee voted articles of impeachment against President Richard Nixon in 1974, but he resigned before the full House could consider them. The primary charge against Nixon was that he had engaged in obstruction of justice with regard to the cover-up of the break-in to the Democratic headquarters at the Watergate building in Washington, D.C. The “smoking gun” that led to Nixon’s resignation was the revelation of a taped conversation where Nixon said that he would tell the FBI not to investigate the Watergate burglary because it was a CIA matter. Such interference with a pending investigation is obstruction of justice.

Obstruction of justice occurs when one “obstructs, influences or impedes any official proceeding.” There is reason to believe that President Trump did just this in trying to stop an FBI investigation into whether Michael Flynn, a Trump campaign aide and briefly a high-level official in the Trump administration, violated federal law in his contacts with Russia. Last week, former FBI Director James Comey revealed that President Trump said that he hoped that the FBI would “let it go” with regard to the investigation of Flynn. Comey said that he felt pressured by the president to end the investigation and that Comey recorded this in a memo that he wrote at the time.

Perhaps more important, President Trump has made statements that he fired Comey precisely to end the investigation about possible illegal activity by Flynn and others. If so, that is obstruction of justice. After firing Comey, President Trump told Lester Holt of NBC “this Russia thing” was on his mind when he fired Comey.

Last Friday, the New York Times reported that President Trump told two Russian officials that Comey was fired to end “pressure” with regard to the investigation concerning Russian influence in the election. President Trump invited Russian Foreign Minister Sergei Lavrov and Russian Ambassador to the U.S. Sergey Kislyak — both of whom are being investigated by the FBI — to a special Oval Office meeting May 10, just a day after firing Comey. It was earlier reported that President Trump leaked classified information to the two Russians in that meeting. According to the New York Times report, the president also advised Lavrov and Kislyak, “I just fired the head of the FBI. He was crazy, a real nut job.” The Times account also noted that the president observed that “I faced great pressure because of Russia. That’s taken off.”

Robert Mueller, who was appointed last week as a special prosecutor, must investigate whether President Trump engaged in the crime of obstruction of justice, as well as whether crimes were committed by officials of the Trump campaign and the Trump administration. If so, it remains unresolved whether a sitting president can be criminally indicted or whether impeachment is the sole remedy.

At this stage, impeachment seems unlikely. Republicans control both the House and the Senate and do not seem disposed to impeach and remove a president of their party. Impeachment is truly extraordinary in American history. But everything about the Trump presidency has been unlikely and even staunch Republicans have expressed great concern about the latest revelations. No longer does talk about consideration of impeachment seem premature.

Erwin Chemerinsky is dean of the UC Irvine School of Law.