Why are the Democrats fighting for witnesses and why are the Republicans fighting against witnesses? Isn't it standard procedure to have witnesses in a trial? Like my title suggests, I'm wondering what the argument is for not having witnesses for the impeachment trial?
One argument that you'll hear against having witnesses is that they will not change the outcome of the trial. There is virtually no chance that the Republican-controlled Senate will vote to remove Trump, and calling witnesses will not change that. There is relatively little debate about the major facts of the case, so having witnesses corroborate those facts even further may not provide any new information.
A big part of Trump's defense is that even if the Ukraine aid freeze was explicitly linked to an investigation of Biden primarily for political gain, that it would not amount to an impeachable offense. For people who share that opinion, Trump himself saying "I used my office for personal political gain" would not change anything, since those individuals don't consider that conduct impeachable. Even if there was smoking-gun testimony confirming that the impeachment managers' charge is exactly what happened, unless 67 senators consider that charge impeachable, proving it is an arguably irrelevant exercise.
The argument is essentially "we all know how this is going to end anyway, so let's not draw it out". The question of "Did he do it?" becomes a lot less important if you've already decided that the answer to "Should he be punished for it?" is "No".
The Republicans don't want to give validity to the process itself by conducting it as something the public would see as a 'trial'. Their narrative is that the impeachment is a purely partisan exercise and retribution against Trump for winning the 2016 election. Calling witnesses would make the impeachment proceedings seem more 'real' and thus implant doubt in voters' minds that maybe Donald Trump did do something bad.
The Democrats, on the other hand, have always wanted a variety of witnesses since November, but few of those witnesses close to the president ended up honoring the subpoenas Democrats sent them. They're hoping that if the Republicans ask these people to testify, partisanship dictates that they're more likely to do so than if the Democrats ask. And if these insiders start testifying, then GOP senators will be forced to directly interact with the truth.
The argument is that they are unnecessary because Adam Schiff confirmed that they had completely and totally proved their case. So why waste any more time on it when the Senate has other business to attend to?
If the Senate majority votes to effectively say "We will take Adam Schiff at his word, and therefore we see no reason to call witnesses", what's the problem?
Sekulow: "After 31 or 32 times you said you proved every aspect of your case... [pauses for response] That's what you said."
Schiff: "We did."
Sekulow: "Well then I don't think we need any witnesses."
It is unnecessary to beat a dead horse.
The argument for more witnesses boils down to "All the facts must come out."
The argument against witnesses is essentially "Given what's charged in the articles of impeachment, more facts is a waste of time."
The decision boils down under the senate rules to a vote, and if 51 of the Senators present vote for to hear witnesses, if the vote count for is only 50, the Senate can move on to other business.
Senator Lamar Alexander, who Democrats had hoped would be the 51st vote for witnesses, announced last night he opposed calling witnesses, explaining:
There is no need for more evidence to prove something that has already been proven and that does not meet the United States Constitution's high bar for an impeachable offense.