Given congress's recent decision to bar witnesses and evidence from being presented as part of President Trump's impeachment trial, are members of the Senate compelled to acquit based on lack of evidence? It seems akin to a prosecutor being prohibited from bringing evidence against an accused criminal. In such a case, the only evidence available to the jury would be the fact that the prosecutor thinks the criminal is guilty, which is clearly insufficient evidence for a conviction.
Impeachment is a political act. There is no crime so heinous for which members of either house could be found criminally or civilly liable for voting no on impeachment. There is also nothing so pure and beneficial which they are forbidden from using as justification for impeachment.
The President could be impeached by the House on the grounds the President was a dead dog, and that judgment could be confirmed by the Senate, with a judgment to either censure or remove the President from office.
An Impeachable Offense is what 1/2 (+1) of the House says it is. To successfully Impeach the President you also need 2/3rds of the Senate to agree. There are no other requirements or limitations.
Trump will be acquitted not because of lack of witnesses or lack of actual criminal actions, but for the exact same reason that he was Impeached, because the people that hold office in the part of the Congress where that decision is made, thought it made political sense or moral sense to do so.
None of them are compelled by anything other than their conscience and their ambition.
The Senate isn't legally compelled to rule either way. If Trump were to come right out and indisputably admit to withholding aid for personal gain, there is no legal requirement for the Senate to convict. Nor is there a legal requirement for the Senate to acquit in the reverse case, in which the accusers all recant their statements.
The Senate can rule however it wants regardless of the evidence. The only thing truly binding them is the next election: "If I vote to acquit, will my constituents re-elect me?"