The best reason to impeach Donald Trump is that he won by cheating in 2016 and he will try to do so again in 2020. Even if the Senate refuses to remove him from office, which would render him ineligible to any public office in the United States in the future, still public hearings that explore his many crimes, including trying to conspire with Russians, whether he succeeded or not, to win the election, should help the public make a more informed choice the next time we have to choose a president.
Remember, way back in 2017, at the beginning of the nightmare that is the Trump administration, one of the first things Trump did was to empanel a commission with the ostensible mission of investigating the allegedly huge problem of voter fraud that was mostly a figment of Trump’s vivid imagination. Recall that he consistently propounded the unsubstantiated claim that between 2 million and 3 million people voted fraudulently in California in 2016.
Insofar as there is any problem with voter fraud in the United States — there isn’t — that problem mostly comes from Republicans. They cheat in numerous ways, by gerrymandering, by voter suppression, and by literally stealing votes.
More specifically to Trump and impeachment, Mueller made a sharp distinction between a criminal investigation and a counterintelligence investigation, expecting that his criminal investigation — which was all he saw himself as pursuing — would turn up information that was important for counterintelligence purposes. Throughout his investigation, FBI agents worked in his office, relaying important counterintelligence information back to the FBI for further action as necessary.
Arguably the worst aspect of Trump’s malfeasance was the attempts that he and his campaign made, however bumbling and ineffective, to get help from Russians, who interfered in the 2016 election to benefit Trump, whether Trump is willing to admit it or not. The line between criminal and counterintelligence is apparently not completely clear, since it might seem to turn on whether the targets are U.S. citizens or foreign nationals, yet Mueller himself indicted a number of Russian individuals and corporations.
But the point remains that Mueller specifically refrained from pursuing the counterintelligence information his investigation produced, which might prove to be the most important information for purposes of ensuring the integrity of U.S. elections.
This is one of many questions the House of Representatives has a duty to answer as part of the investigation it should pursue explicitly under the rubric of impeachment.