It can be argued that most presidents get around to committing a high crime or misdemeanor during their term in office. Just in this century, George W. Bush authorized the torture of terror suspects and warrantless surveillance of Americans, while Barack Obama controversially had an American terror suspect executed by drone attack. Those were terrible acts, but they at least were arguably undertaken in the name of American national security, on behalf of the citizenry.
So it isn’t the case that simple policy differences, or mere politics, have been criminalized by the impeachment process. Trump wasn’t put on trial for bypassing Congress to build his border wall. Nixon wasn’t forced from his job because he bombed Cambodia. In both cases, they faced impeachment precisely because they abused their authority to benefit themselves and their own political purposes.
If Republicans want to be reasonable, that will be the precedent they take from this last sordid year, and not simply that it is possible to impeach a president for any old reason just because you have majorities in Congress. Of course, reason appears to be in short supply within the GOP these days. But if frivolous impeachments become de rigueur the next time the party takes power, it will not be because Democrats opened Pandora’s Box — it will be a conscious choice by Graham and his Trumpist cronies.