1. Adverse public opinion
2. Bad economy
3. Hostile media
4. “Incontrovertible evidence” of illegal activity
In his book about the Clinton impeachment, Bob Woodward quotes Senator Fred Thompson, a Republican who voted against removal, who cited the four factors above as necessary, and possibly sufficient, for impeachment and removal of a president. Thompson was generalizing from one (Nixon) or possibly two (Nixon and Johnson) data points, but his theory turned out to hold for Clinton as well.
The relationship between these factors is complicated; they are surely not independent of each other. Nixon enjoyed extremely high standing in the public (>60%) coming off his reelection, bolstered by significant foreign policy accomplishments. It was the gradual disclosure of increasingly incontrovertible evidence of illegal activity that destroyed his standing among the public. The bad economy and hostile media surely did not help. And while the media had always been hostile to Nixon, as it has been with Trump, the disclosure of evidence made it more hostile—as stalwart Republican papers like the Chicago Tribune eventually turned against him as a result of the disclosures.
Clinton, by contrast, remained popular throughout the revelations that led to his impeachment, and throughout the impeachment proceedings as well (#1). The economy was strong (#2). And while there was incontrovertible evidence of perjury and obstruction of justice (#4), the underlying behavior—adultery—was not considered very serious by the public. People disapproved of Clinton’s behavior but did not think a president should be impeached over it. We should amend #4 to state “illegal activity beyond the cover-up itself”—burglary, espionage, violation of people’s rights, rather than adultery. Clinton also faced a hostile media, though maybe not as hostile as the media faced by Nixon and Trump.
Trump is the least popular president of the modern era. While other presidents have done worse in the polls from time to time, no president has done so badly in the honeymoon period at the start of the presidency and during a period of economic prosperity and international peace. Trump’s approval rating is worse than Nixon’s even after the Senate Watergate hearings began in May 1973. At that time, less than 20% of Americans supported impeachment of Nixon. That number would not rise above 40% until almost a year later, after the Saturday Night Massacre and additional revelations. More than 40% of the public already wants Trump to be impeached.
So much for #1. We can also pass over #3. If Thompson is right, the only question is if (and when) the economy tanks, and if (and when) the “incontrovertible evidence of illegal activity” emerges. Economists estimate a 10-20% probability of a recession in 2018. If you think that the probability of incontrovertible evidence coming to light is 100%, then we can understand why prediction markets suggest a 20% probability of impeachment in 2018. The probability of impeachment is just the probability of recession (to a first approximation).
But that leaves the question of #4. At what point is evidence “incontrovertible” (and does it really need to be?), and how serious does the illegal activity need to be (and does it even need to be illegal?)? For both Nixon and Clinton, the evidence really was incontrovertible. Will we ever reach that stage for Trump? If Trump or top aides actually colluded with the Russians, and aides testify under oath, will that be enough? Do we need a memo? Audio? Video? What if Trump merely engaged in shady financial transactions with Russians long before he ran for election? Hard to say.
One thing that places Trump in a category different from Nixon and Clinton is that no one thought Nixon or Clinton was incompetent. Amoral, yes; impulsive, in Clinton’s case; but not incompetent. This is an additional factor to take into account, one that did not even occur to Thompson. If Trump convinces us all that he welcomes the prospect of a nuclear war with North Korea, then one way or another he’s going to get pushed out.