Only three American presidents have suffered the indignity of being impeached: Andrew Johnson in 1867; Bill Clinton in 1998; and now Donald Trump.
One manifestation of the U.S. political system’s “checks and balances” was that both Johnson and Clinton escaped being removed from office when their cases were heard in a Senate trial. In the case of Trump, the GOP-controlled Senate’s vote not to seek the testimony of additional witnesses (such as John Bolton and Lev Parnas) likely ensures Trump will remain in office.
The United States was founded as a nation that repudiated the absolute centralized power associated with monarchies. The American political system is based upon power sharing between the three branches of government-- Executive, Legislative, and Judicial. Consequently, any attempt by one branch to circumvent the power of another threatens the entire system’s viability.
Besides Ukraine, Donald Trump’s actions throughout his presidency indicate he sees himself as above the law. In fact, he is the only president in U.S. history who not only has expressed admiration for authoritarian leaders around the world, but has publicly disdained parts of the Constitution that don’t validate his personal beliefs. Astonishingly, this apparently makes no difference to Republicans in Congress. If this isn’t a scary scenario, I don’t know what is.