The Constitution authorizes presidential impeachment if the president can be shown to have committed "treason, bribery, or other high crimes and misdemeanors."  Democrats are ignoring this language and trying to recreate a parliamentary system.

Since the Nation's founding, it's been understood that the term "other high crimes and misdemeanors" refers to indictable crimes, just as treason and bribery do:

As you can read in the journals of past infamous impeachment trials, Founding Fathers, respected jurists, and legal scholars each argued near the very founding of our country: "[n]othing is impeachable that is not also indictable." (Source: Hinds' Precedents, Volume 3, Chapter 72, "The Impeachment and Trial of Samuel Chase," published by the U.S. Government Publishing Office[.)]

The Democrats, however, have no indictable offense against President Trump.  Neither "abuse of power" nor "obstruction of Congress" forms the basis for a criminal complaint.

Three days of Democrat lectures establish that their problem with Trump is political, not criminal.  Trump defied the foreign policy establishment's often left-leaning, always atrophied, and usually wrong approach to foreign policy.  He also sneered at the House's demands for documents and witnesses that implicated executive privilege pointing out that they did not use a formal vote to initiate impeachment proceedings and would not take the matter to the Supreme Court.  In other words, he's opposed their plans for a socialist, globalist America and made them look weak.

If the Democrats are not engaged in an actual impeachment, then what exactly are they doing?  The answer to that comes from two legal thinkers, one writing about Nixon's impeachment and the other about Trump's.

Writing at Power Line, Steven Hayward discusses an essay that lawyer and legal scholar George Anastaplo wrote in February 1974, entitled "Impeachment and Statesmanship."  The essay states in relevant part (emphasis added):

Certainly, "loss of confidence" should not provide the basis for impeachment. Ours is not — and probably should not be turned into — a parliamentary system. ... We should take care not to turn "loss of confidence" (or even "a pattern of misconduct") into an impeachable offense. A President should sometimes be quite unpopular, even for a long period of time, if the common good is to be served. The purpose of Presidential impeachment, it should be remembered, is not to serve as a recount of the last election, or as a censure of elected officials or as a way of gratifying public opinion.

Fast-forward 46 years, and Josh Blackman, a constitutional law professor at the South Texas College of Law in Houston, writes in Friday's New York Times (!) that "Trump Acts like a Politician. That's Not an Impeachable Offense."  The gist of Prof. Blackman's essay is that presidents, even as they've put policies in place, have always had an eye to their political fortunes.  Sometimes it's obvious ("what's good for the country is good for my re-election"), and sometimes it's subtle, as when Lincoln temporarily weakened the Union Army in 1864 to send Indiana troops home to vote, ensuring that Indiana would remain politically loyal to the union.

Just as Blackman explained what impeachment is, he also explained what it is not (emphasis added):

[R]egrettably, the House of Representatives has transformed presidential impeachment from a constitutional parachute — an emergency measure to save the Republic in free-fall — into a parliamentary vote of "no confidence."

And there's that idea again: we do not have a parliamentary system.

In a parliamentary system, people do not vote for the prime minister.  Instead, they vote for party representatives.  Whichever political party gets the most representatives in parliament gets to appoint that party's "prime" minister, who then gains executive control over the government.  If the policies aren't working, parliament — including, sometimes, the prime minister's own party — will have a vote of "no confidence," automatically triggering new elections.  The people will return to the polls to again choose representatives who will, in turn, once again choose a "prime" minister.

In America, though, the people choose their prime minister or, as we say, "the president."  The Constitution does not allow Congress to dismiss the people's choice because it dislikes the president's policies.  Instead, our system says that if the president does not commit an indictable crime, Congress has no say whether he stays in office.  After a president's first four years in office, it's the people who get to vote on that president's policies and performance.

The Democrats have forgotten this important fact about our constitutional structure.  Constitutionally minded voters should turn out in November 2020 to remind them.