The first general school of thought is that the standard enunciated by the Constitution is subject entirely to whatever interpretation Congress collectively wishes to make:

"What, then, is an impeachable offense? The only honest answer is that an impeachable offense is whatever a majority of the House of Representatives considers it to be at a given moment in history; conviction results from whatever offense or offenses two-thirds of the other body considers to be sufficiently serious to require removal of the accused from office..." Congressman Gerald Ford, 116 Cong. Rec. H.3113-3114 (April 15, 1970).

The fourth view is that an indictable crime is not required, but that the impeachable act or acts done by the President must in some way relate to his official duties. The bad act may or may not be a crime but it would be more serious than simply "maladministration." This view is buttressed in part by an analysis of the entire phrase "high crimes or misdemeanors" which seems to be a term of art speaking to a political connection for the bad act or acts. In order to impeach it would not be necessary for the act to be a crime, but not all crimes would be impeachable offenses.

How Congress Sets the Rules for Impeachment

Both the U.S. House of Representatives and the U.S. Senate have the right to make their own rules governing their procedure, and to change those rules. Under current rules, the actual impeachment inquiry begins in the Judiciary Committee of the House of Representatives. That Committee holds hearings, takes evidence, and hears testimony of witnesses concerning matters relevant to the inquiry. Typically, as occurred in the case of President Nixon, there will also be a Minority Counsel who serves the interest of the party not controlling Congress.

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Whoever happens to be currently serving in the Congress at the moment get to decide, for themselves, what constitutes an impeachable offense and they also get to decide, for themselves, what procedures they adopt to investigate allegations.

Lying to Congress is indeed a crime, but it is up to the Congress itself (and no other authority) to decide whether or not to pursue allegations of perjury.

This goes for BOTH PARTIES. It seems that people are quick to forget about Bill Clinton who was dragged through a full impeachment trial for hair-splitting the not-legally-defined phrase "sexual relations".

The Congress was perfectly within their constitutional rights to decide if getting a BJ (while not an actual CRIME) was considered "an impeachable offense" (presumably because it was behavior "unbecoming" to a sitting president).

THE EXACT SAME STANDARDS-OF-EVIDENCE APPLY TODAY.

Asking foreign governments to interfere with an election would seem to be at a minimum "grossly inappropriate" and very likely specifically criminal.

Even FOX manages to get their facts straight on this one,

Furthermore, if your only "defense" is that "Joe Biden did the same thing" and you are arguing that what Joe Biden did WAS A CRIME, then you are admitting that YOU ALSO COMMITTED A CRIME.

Imagine an alleged murderer complaining that "OJ Simpson did the same thing, you should be investigating HIM".

Doesn't that just make you look MORE GUILTY?