Most Republicans oppose the impeachment and removal of President Trump because they don’t think he has committed any high crimes and misdemeanors. As Jim Geraghty notes, James Comey opposes it for a different reason: It would embitter Trump’s supporters, who would consider it a “coup.”
That argument makes a lot less sense than the Republican one, which, granting its premise of no impeachable offenses, is airtight. A president can’t be removed from office without a very high degree of consensus in the country–much higher than is required to elect a president in the first place. Every Senate Democrat and twenty Senate Republicans would have to agree before it could happen. The procedural barrier against removal from office is extremely high, and so there’s no real need to have a norm against it, let alone a norm based on its potential to create division.
I suspect that a lot of Democrats have a clearer sense of these issues than Comey. They’re against starting down the road to impeachment and removal because it is unlikely to succeed and might play into Trump’s hands, thereby increasing his time in office. That view, too, makes more sense than Comey’s.