In the attempt to impeach Supreme Court Justice William O. Douglas, then Congressman (later President) Gerald Ford said this:
“The only honest answer is that an impeachable offense is whatever a majority of the House of Representatives considers 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. Again, the historical context and political climate are important; there are few fixed principles among the handful of precedents.” 
While this may offend a belief in the separation of powers, Mr. Ford’s statement is the absolute truth. There is no appeal from a conviction on impeachment. If the House of Representatives decides to impeach, and the Senate decides to convict, a public officer for having smelly feet, there is no way to correct it.
The U.S. Constitution provides that the President can be “be removed from Office on Impeachment for, and Conviction of, Treason, Bribery, or other high Crimes and Misdemeanors.”  This appears to mean that the President can only be removed by the impeachment process if he has committed a crime of some sort. But it’s not that simple.
“The phrase ‘high crimes and misdemeanors’ in the context of impeachments has an ancient English history, first turning up in the impeachment of the Earl of Suffolk in 1388. Treason is defined in the Constitution. Bribery is not, but it had a clear common law meaning and is now well covered by statute. ‘High crimes and misdemeanors,’ however, is an undefined and indefinite phrase, which, in England, had comprehended conduct not constituting indictable offenses. Use of the word ‘other’ to link ‘high crimes and misdemeanors’ with ‘treason’ and ‘bribery’ is arguably indicative of the types and seriousness of conduct encompassed by ‘high crimes and misdemeanors.’ Similarly, the word ‘high’ apparently carried with it a restrictive meaning.
“Debate prior to adoption of the phrase and comments thereafter in the ratifying conventions were to the effect that the President (all the debate was in terms of the President) should be removable by impeachment for commissions or omissions in office which were not criminally cognizable. And in the First Congress’s ‘removal’ debate, Madison maintained that the wanton dismissal of meritorious officers would be an act of maladministration which would render the President subject to impeachment. Other comments, especially in the ratifying conventions, tend toward a limitation of the term to criminal, perhaps gross criminal, behavior. The scope of the power has been the subject of continuing debate.” 
Research and you will find scholarly voices claiming that the Framers meant something specific by the phrase “high Crimes and Misdemeanors,”  but in this case erudition tends to conceal the actual lack of specificity. And the fact that there were differing views on the Constitutional language at the time of ratification reveals that the phrase was not such a subject of universal legal understanding as some pundits would have us believe. The most common reference source for the Common Law at the time, Blackstone’s Commentaries on the Laws of England, didn’t even use the phrase. 
Still, it cannot be denied that there have been impeachments for behavior not cognizable by the criminal law. In his Commentaries on the Constitution of the United States, Supreme Court Justice Joseph Story said this:
“The next inquiry is, what are impeachable offences? They are ‘treason, bribery, or other high crimes and misdemeanors.’ For the definition of treason, resort may be had to the constitution itself; but for the definition of bribery, resort is naturally and necessarily had to the common law; for that, as the common basis of our jurisprudence, can alone furnish the proper exposition of the nature and limits of this offence. The only practical question is, what are to be deemed high crimes and misdemeanors? Now, neither the constitution, nor any statute of the United States has in any manner defined any crimes, except treason and bribery, to be high crimes and misdemeanors, and as such impeachable. In what manner, then, are they to be ascertained? Is the silence of the statute book to be deemed conclusive in favor of the party, until congress have made a legislative declaration and enumeration of the offences, which shall be deemed high crimes and misdemeanors? If so, then, as has been truly remarked, the power of impeachment, except as to the two expressed cases, is a complete nullity; and the party is wholly dispunishable, however enormous may be his corruption or criminality. It will not be sufficient to say, that in the cases, where any offence is punished by any statute of the United States, it may, and ought to be, deemed an impeachable offence. It is not every offence, that by the constitution is so impeachable. It must not only be an offence, but a high crime and misdemeanor….
“Again; there are many offences, purely political, which have been held to be within the reach of parliamentary impeachments, not one of which is in the slightest manner alluded to in our statute book. And, indeed, political offences are of so various and complex a character, so utterly incapable of being defined, or classified, that the task of positive legislation would be impracticable, if it were not almost absurd to attempt it….
“Congress have unhesitatingly adopted the conclusion, that no previous statute is necessary to authorize an impeachment for any official misconduct; and the rules of proceeding, and the rules of evidence, as well as the principles of decision, have been uniformly regulated by the known doctrines of the common law and parliamentary usage. In the few cases of impeachment, which have hitherto been tried, no one of the charges has rested upon any statutable misdemeanors. It seems, then, to be the settled doctrine of the high court of impeachment, that though the common law cannot be a foundation of a jurisdiction not given by the constitution, or laws, that jurisdiction, when given, attaches, and is to be exercised according to the rules of the common law; and that, what are, and what are not high crimes and misdemeanors, is to be ascertained by a recurrence to that great basis of American jurisprudence.” 
Smelly feet, in other words.
But “Story’s implicit rejection of a perhaps more plausible construction of ‘high Crimes and Misdemeanors’ may reflect the confluence of conflicting realities represented by the dangers of partisanship and the failure of the phrase’s more commonly understood meaning to reach universally accepted grounds for removal such as insanity. Contrary to the weight of authority but consistent with much of the evidence, the term ‘high Crimes and Misdemeanors’ may have been intended to simply mean all crimes. Blackstone seems to indicate that the terms ‘high crimes and low crimes,’ ‘felonies and misdemeanors,; and ‘high crimes and misdemeanors’ were thought synonymous. By the time Story’s Commentaries first appeared in 1833, however, the young Republic had experienced the impeachment and conviction of Judge Pickering, necessitated by his apparent insanity, and an abortive attempt by some in the case of Justice Chase to use impeachment to purge the federal bench of the appointees of their political opponents….How better to avoid these distasteful alternatives than to shroud the phrase ‘high Crimes and Misdemeanors’ in an accommodating veil of uncertainty capped by the sage observation that…the identification of the circumstances that warrant an official’s removal must be made on an ad hoc basis?” 
With the United States being as politically divided as it is now, there is concern afoot that the current impeachment inquiry into President Trump is a purely partisan endeavor. So convinced are some that it is a common expectation that the Democratic House will vote to impeach, but that the Republican Senate will acquit him. There does not appear to be, for many, the expectation that the procedure will be anything but an exercise of party politics; and for some that doesn’t even seem to be an aspiration.
The legal response of the parties is predictable. The Democrats will claim that proof of an actual crime on the part of the President isn’t necessary for an impeachment and conviction, and the Republicans will say that it is. It will behoove the Democrats, therefore, to actually make out a crime if they expect to have any success in gaining a conviction in the Republican led Senate.
The suspicion is that Trump was conditioning needed military aid to Ukraine on whether its president agreed “to investigate the son of former vice president Joe Biden” in connection with business dealings in that country.  It is beyond question that Trump had held up almost $400 million in military aid, previously authorized by Congress, at least a week before his now infamous telephone call with the Ukrainian president, wherein he applied pressure to resurrect the investigation.  And this, even though “Ukraine’s former top law enforcement official says he repeatedly rebuffed demands by President Trump’s personal lawyer to investigate Joe Biden and his son, insisting he had seen no evidence of wrongdoing that he could pursue.” 
Now, intimidating, coercing, threatening, or harassing a foreign official, or attempting to do so, is a species of extortion under federal statute.  Here, President Trump is alleged to have threatened the Ukrainian president that U.S. military aid would continue to be withheld unless Ukraine reopened an investigation that Trump hoped would yield him political benefit. The charge is that Trump tried to coerce action on the part of the Ukrainian criminal justice system. If proven, this would be a clear act of extortion beyond the reach of sophistry.