In September, a law professor argued that lawsuits against Trump University had already laid the groundwork for an impeachment case. A history professor who has accurately predicted every presidential election since 1984 has said Trump's impeachment is imminent.
But calls for impeachment escalated this week following a series of revelations. Last week, Trump fired FBI Director James Comey and later linked the decision to the FBI investigation into possible collusion between Trump associates and Russia. On Monday, the Washington Post reported that Trump had shared classified intelligence with Russian officials during a meeting in the Oval Office. (White House officials have denied that report in broad strokes but have not rebutted specifics of the report.) And on Tuesday, the New York Times, citing a memo written by Comey, reported that Trump asked Comey to shut down the investigation into former national security adviser Michael Flynn in February. The White House has denied the Times report.
While some Democrats have been raising questions about impeachment for months, party leaders are still approaching the subject with caution.
"If the facts warrant, there will be time to talk about that," Rep. Steny Hoyer, the House Democratic whip, said Tuesday when asked about impeachment, which he called "one of the most serious matters in a democracy." In remarks reported by The Hill, he said: "It ought not be taken lightly, it ought not to be taken politically."
House Minority Leader Nancy Pelosi on Monday cautioned against calling for impeachment prematurely.
"What are the facts that you would make a case on?" she said during a CNN interview. "What are the rules that he may have violated? If you don't have that case, you're just participating in more hearsay."
The process of impeaching an American president, much less removing them from office, is quite challenging, as can be seen from the impeachment of President Bill Clinton and the idle chatter of impeaching Presidents George W. Bush and Barack Obama.
Here's a closer look at how hard the process actually is.
Impeachment talk would need to be taken seriously
Bush and Obama both faced idle impeachment threats that never amounted to anything. People who now believe there's a serious case for impeaching Trump will have to overcome the reputation established by those who raise the specter of impeachment merely to demonstrate political opposition.
"That makes it harder to separate the wheat from the chaff, so to speak, to figure out what’s really the substantive conduct and the substantive problem," said Michael Gerhardt, a constitutional law professor at the University of North Carolina at Chapel Hill. "We can’t rely on political rhetoric to guide us because political rhetoric is so overheated."
Critics would need to settle on one argument
The Constitution states that a president can be impeached if convicted of "treason, bribery or other high crimes and misdemeanors." Those looking to impeach Trump would need to show he has done something that falls into one of those categories — which requires more evidence.
While treason and bribery are defined by the Constitution and by federal law, "high crimes and misdemeanors" is a less specific charge. Gerhardt said the framers intended it to refer to "political crimes," including abuses of power or other offenses against the United States. "They don’t have to be technically criminal things — things for which someone could go to prison — but they do have to reach a certain level of seriousness," he said.
Some of Trump's critics have argued that his business dealings are in violation of the Constitution's Emoluments Clause, which prohibits the President from accepting gifts from foreign leaders or governments. Others, including Waters, have argued that the ties between Russia and Trump's team are signs of wrongdoing. Christopher Peterson, a University of Utah law professor, maintains that the Trump University lawsuits provide grounds for impeachment and thinks there's already a "fairly solid" case to be made.
But for Trump's opponents to realistically pursue impeachment, they would likely need to focus on investigating one offense and making a specific, formal accusation of wrongdoing.
There would need to be more evidence
Right now, arguments for impeachment are resting on potentially flimsy claims. "The critical thing that those congressmen will have to show is that he’s done something that will qualify as treason, bribery, or other high crimes and misdemeanors," Gerhardt said. "That’s the threshold."
While there are mounting questions about potential coordination between Trump's campaign and Russian interests, no concrete conclusions have been reached.
"So far there hasn’t been a clear smoking gun," Peterson said.
"I do think there are certain ways in which he’s conducting himself in office that will come under scrutiny," said Gerhardt, who served on the transition team for Clinton's Justice Department and later testified before the House Judiciary Committee on Clinton's impeachment as a shared witness. "To what end does that scrutiny end up producing any evidence or concern about how he’s exercising power?" He said it's too early in Trump's presidency to know.
The House would need to decide there are grounds for impeachment
Impeachment proceedings begin in the House of Representatives, where lawmakers can introduce an impeachment resolution or a resolution authorizing an investigation into whether grounds for impeachment exist. If a House committee determines that there are grounds for impeachment, a resolution with a formal accusation of misconduct is presented to the full House for a vote.
In order to impeach a president, that resolution must pass the House by a simple majority. When the President's own party has control of Congress — as Republicans do now — that's a difficult bar to clear. If a vote were to take place today, when there are five vacancies in the House, all 193 Democrats and 23 Republicans would need to vote for impeachment in order for it to pass.
"Impeachment is always difficult. It’s designed to be difficult," Gerhardt said. "That’s the nature of the process, the nature of the constitutional design."
Only two presidents in U.S. history have been impeached: Clinton in 1998 on charges of lying under oath to a federal grand jury, and Andrew Johnson in 1868 on charges of violating the Tenure of Office Act by firing the Secretary of War. Both Clinton and Johnson were Democrats who faced a Republican-controlled Congress, and both were still acquitted in the Senate because opponents failed to gather enough votes in the upper chamber. (Richard Nixon resigned before he could be impeached.)
"The key committee chairs in the House of Representatives are reluctant to take on the President of their own party," Peterson said of Trump's situation. "Whether or not they will act depends on how much political pressure is brought to bear on them."
The Senate would need to find the President guilty
In order to actually be removed from office, the President must then be convicted by a two-thirds vote in the Senate.
When the President's own party controls the chamber, that's unlikely to happen unless there is evidence of serious misconduct. Based on the current party makeup in the Senate, 19 Republicans would have to side with all 46 Democrats and two independents in order to remove Trump.
Even Clinton and Johnson, who faced chambers controlled by the opposing party, didn't provoke a consensus that strong. Both served out the remainder of their terms.
There would need to be public support for impeachment
Impeachment would probably need to be popular in order for Congress to act on it. Trump's approval ratings continue to be historically low for a new president, but it's not yet clear how Americans would feel about impeachment. A survey released May 16 by the left-leaning Public Policy Polling found that more voters (48%) support impeaching Trump than oppose it (41%). In March, voters were split: 44% supported impeachment, while 45% opposed it.
An impeachment trial could backfire. During Clinton's impeachment proceedings, public opinion of Republicans fell, while Democrats and Clinton experienced a surge. When asked whether they wanted Clinton or the GOP "to have more influence over the nation," Americans were evenly split between the two in September 1998, according to a CNN/USA Today/Gallup poll. But by the time Clinton was impeached in December of that year, the gap had widened significantly. While 60% said they wanted Clinton to have more influence, just 31% said the same for the GOP.
The proceedings also had the unintended effect of sending Clinton's approval ratings to an all-time high.