The impeachment in the House was a political debate - not really a legal one. The rules that apply in a regular trial did not apply there. Is the same true of the Senate trial or is that more of a legal process?
Either way, what are the ramifications of this?
In UK, impeachment process became part of the Common Law. However, in the US constitution, founding fathers made some explicit decisions to do things differently. Cornell Law School provides some annotations on the US constitutional history here.
In UK, conviction on an impeachment has resulted in fine, imprisonment, and even execution (as in the case of William Laud). In contrast, under the US Constitution, the penalties under of an impeachment cannot extend further than removal from the office. It seems the US founding fathers had a specific purpose here:
Under the U.S. system, someone who has been impeached and removed by the Senate can later be tried and convicted in a separate court of law for the exact same act(s). If the impeachment were tried as a matter of law, this would be impermissible as double-jeopardy under the law.
In terms of the current controversy around Trump's impeachment, it would be unreasonable to expect the Senate trial to be any less political than what happened in the House. However, due to the impeachment rules of the Senate, the debate will all be done in closed session.
The way the trial is proceeding, it's definitely a political affair, not a legal one. A legal trial would be focused on answering questions such as "did Trump pressure Ukraine to investigate a political rival? If so, does this qualify as an impeachable offense?" Partisanship shouldn't factor, because the question doesn't involve Trump's political party at all. In other words if Trump were to switch from the GOP to the Democrats, the Democrats in the Senate should still vote to impeach him.
In practice, what's happening is (source):
Afterward, Republican senators met behind closed doors to discuss calling witnesses including Bolton, but said as they emerged that there was no resolution on the matter.
To even do this kind of closed-door, Republican-only meeting, they have to be viewing the trial as a partisan affair.
Schumer made a fresh appeal for four Republican senators - the number needed for a majority - to join Democrats in voting to call witnesses. Schumer also indicated Democrats would reject any effort at a so-called witness swap with Republicans.
This indicates the Democrats are also viewing the trial as a political, not legal, affair - in a law-based trial, witnesses would be called if they knew something about the topic being discussed, not as part of a "witness swap" deal.
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:
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.
