Trump, Impeachment and the Constitution
The Constitution is the supreme law of the land, thus, all government behavior must conform to it. It is, of course, notwithstanding its supremacy, an imperfect document. Its original iteration in 1789 -- and even after the addition of the Bill of Rights in 1791 -- implicitly recognized slavery, permitted the states to limit voters to adult white landowning men and did not require the states to protect personal liberty.
Under the Constitution, impeachment -- a charge accusing a present or former federal officeholder of paramount wrongdoing -- can only be had if the charge is for a criminal act. The constitutional language -- "treason, bribery, or other high crimes and misdemeanors" -- means the criminal act must be grave and strike at the security of the republic.
President Andrew Johnson was impeached in 1868 for firing his secretary of war, in defiance of a statute -- which was no doubt unconstitutional -- prohibiting him from doing so. Whatever he did, it was not criminal, nor was it the type of behavior that affected the security of the republic. He was a southerner who was despised by the Radical Republicans who ran the government after Lincoln's death. He was acquitted in the Senate by one vote.
President Bill Clinton was impeached in 1998 for lying under oath about a private consensual sexual relationship. There was a crime -- perjury -- but it was not of the species of crime that the framers contemplated when they wrote "other high crimes and misdemeanors." It surely lacked the gravity of treason and bribery.
Clinton was despised by the Republicans who ran the House. Either they believed his presidential perjury about consensual sex was a serious threat to the republic or they used it as an instrument to embarrass him. He was acquitted in the Senate.
Both of President Donald Trump's impeachments were based on presidential behavior that either sought to use the instruments of government for personal gain (his 2019 impeachment was based on favors he requested from the president of Ukraine in return for military aid) or threatened to interrupt the constitutional duties of the Congress to prevent it from certifying the results of the 2020 presidential election. He, too, has been despised by the Democrats that have run the House, and they looked for and found mechanisms to tarnish his legacy.
Add to this the unique -- though not unprecedented -- feature of Trump's second impeachment trial: It occurred while he was a private citizen. He was acquitted both times in the Senate.
The point of this brief presidential impeachment history is that the Constitution is flexible and gives opportunities for congressional mischief. This is particularly so in the cases of impeachment. The House of Representatives and the Senate write their own rules and procedures -- for impeachment and for all else. The procedures once enacted are not subject to challenge in the other house or in the courts.
Congress' internal workings -- though biased, unfair and even bullying -- are only subject to appeal to the voters at the next election.
Impeachment is far more a political mechanism than a legal one. In Johnson's case, there were no hearings and there was next to no debate in the House before the vote to impeach. In Trump's recent impeachment, he was denied due process as well, since there were no House hearings. In Clinton's case and Trump's first case, there were extensive due process hearings at which evidence was developed and challenged, and both presidents were represented by counsel.