This is an odd question, because it presumes that there is some sort of split between "legal" and "political".
The trial is legal because it is directly constitutional. It is also political, and it was always intended to be. The court of impeachment is designed only to answer a political question: has the officer committed "treason, bribery, and other high crimes and misdemeanors". This may seem like an odd thing to call "political", but first note that these are deliberately undefined legal terms. Hamilton's Federalist Essay 65 essentially lays out the thought-process.
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.
Therefore, this cannot be a court of law in the traditional sense. Furthermore, the strictly legal, criminal questions would be answered only by a separate court after an impeachment has taken place. This is also deliberate:
Could the Supreme Court have been relied upon as answering this description? ...
The punishment which may be the consequence of conviction upon impeachment, is not to terminate the chastisement of the offender. After having been sentenced to a perpetual ostracism from the esteem and confidence, and honors and emoluments of his country, he will still be liable to prosecution and punishment in the ordinary course of law. Would it be proper that the persons who had disposed of his fame, and his most valuable rights as a citizen in one trial, should, in another trial, for the same offense, be also the disposers of his life and his fortune? Would there not be the greatest reason to apprehend, that error, in the first sentence, would be the parent of error in the second sentence? That the strong bias of one decision would be apt to overrule the influence of any new lights which might be brought to vary the complexion of another decision? Those who know anything of human nature, will not hesitate to answer these questions in the affirmative...
The questions before the Senate are:
- whether "treason, bribery, and other high crimes and misdemeanors" have taken place (in the minds of the senators), and
- whether this merits removal from office.
The only punishment that can be awarded is a political punishment (removal from office), so the question is then, in the first part, factual, and in the second part, political.
None of this is a mistake; all of that is what was intended by the founding fathers and outlined in the documents that have been passed down.