While I am not a lawyer, this is fairly well settled in US constitutional theory. The framers thought of impeachment is something very much like a legal process, which is shown, among other things by the procedure in the Senate being referred to in the Constitution as a "trial" and a vote to remove being described as a "conviction". The use of the term "High Crimes and Misdemeanors" also suggests a legal process.
In The Federalist, Hamilton took the view that the trial was to determine "guilt or innocence", but would be inherently political.[A]
But there is no supervising authority, and the Congress (and each house of it) is a very political body. Procedures for drafting and passing Articles of impeachment in the House are whatever the House chooses them to be on each particular occasion. Procedures and rules of evidence in a Senate trail are whatever the Senate chooses them to be on each occasion. There is no standard format which is binding, or even customary. Thus in practice an impeachment will be a political issue whenever the Congress treats it as one.
Early in the history of the US there was an opinion among some legislators and politicians that an impeachment was simply a statement that "we want your offices to give them to better men"[B], suggesting that it be treated much like a vote of no-confidence in the UK parliament. In the impeachment of Justice Chase the Senate did not vote to convict, and this was held to establish the principle that actual wrong-doing, not mere political dispute, must be the basis of any impeachment.
In the impeachments of Federal Judges, the procedure has been essentially legal, with accusations of actual crime, such as bribery or malfeasance in office, being the basis of impeachments, and Senate trials being quite similar to criminal trials.
This was perhaps particularly apparent in the impeachment and trial of Justice Chase.
The articles of impeachment drafted in the case of president Nixon also looked very much like an indictment for ordinary crime. The trial never took place, but discussions of its format at the tiem suggested that it would have also been somewhat like a criminal trial.
But the impeachments and trials of Presidents Clinton and Trump had a much more political aspect to them. While accusations were made of actions alleged to be not just politically unacceptable but unlawful, the Senate essentially inquired not into the truth of the accusations, but into whether they were improprieties deserving of removal at all. This was essentially a political, not a legal decision, and was made in a quite partisan way in each case.
The US Supreme Court has held that it has no authority to approve or disapprove the outcome of any impeachment proceeding (not does any other federal court). An "impeachable offense" is whatever the House choose to list in an Article of Impeachment, and a "removable offense" is whatever the Senate chooses to vote for removal on the basis of.
Thus those preparing proposed articles or advocating them in a Senate trial will be as legalistic or political as they think the occasion demands.
Notes and sources
The Federalist on Impeachment
[A] Essay number 65 of The Federalist (by Alexander Hamilton) begins:
THE remaining powers which the plan of the convention allots to the Senate, in a distinct capacity, are comprised in their participation with the executive in the appointment to offices, and in their judicial character as a court for the trial of impeachments. As in the business of appointments the executive will be the principal agent, the provisions relating to it will most properly be discussed in the examination of that department. We will, therefore, conclude this head with a view of the judicial character of the Senate.
A well-constituted court for the trial of impeachments is an object not more to be desired than difficult to be obtained in a government wholly elective. The subjects of its jurisdiction are those offenses which proceed from the misconduct of public men, or, in other words, from the abuse or violation of some public trust. They are of a nature which may with peculiar propriety be denominated POLITICAL, as they relate chiefly to injuries done immediately to the society itself. The prosecution of them, for this reason, will seldom fail to agitate the passions of the whole community, and to divide it into parties more or less friendly or inimical to the accused. In many cases it will connect itself with the pre-existing factions, and will enlist all their animosities, partialities, influence, and interest on one side or on the other; and in such cases there will always be the greatest danger that the decision will be regulated more by the comparative strength of parties, than by the real demonstrations of innocence or guilt.
What, it may be asked, is the true spirit of the institution itself? Is it not designed as a method of NATIONAL INQUEST into the conduct of public men? If this be the design of it, who can so properly be the inquisitors for the nation as the representatives of the nation themselves?
It seems that Hamilton's ideal was that of a legal proceeding, a court, which was to determine "guilt or innocence". But he was well aware of th importance that political factors (and factions) might assume in any impeachment trial, and regarded this as inherent in the nature of such a proceeding. Most constitutional scholars seem to have followed this line of thought.
[B] Senator William Branch Giles, leader of the Senate forces favorable to the Administration of President Jefferson, is reported by John Quincy Adams to have said:
Impeachment was not a criminal prosecution ... and a removal by impeachment was nothing more than a declaration by Congress to this effect: "You hold dangerous opinions, and if you are suffered to carry them into effect, you will work the destruction of the Union. We want your offices for the purposes of giving them to men who will fill them better."
quoted from Memoirs of John Quincy Adams (J.P.Ipperncot & Co; 1874) Volume 1, page 322;
quoted in Grand Inquests By Chief Justice William H. Rehnquist (William Morrow & Co; 1993) page 27 (Chapter 1)
The Wikipedia article on Giles (linked above) says:
Giles strongly advocated the removal of Justice Samuel Chase after his impeachment, urging the Senate to consider it as a political decision (as to whether the people of the United States should have confidence in Chase) rather than as a trial.