Trump wants to get this over with fast, have no further evidence out in the public and resume his nonsensical accusations against his enemies. He does not care if the trial confers “legitimacy” because his cult bestows legitimacy on him no matter what he says or does.
When facts come out down the road to reaffirm his guilt, he’ll say witnesses are lying or the Senate said his conduct was “perfect.” He has given up trying (or never tried) to get nonwhites, college-educated voters or suburban women in his corner. He believes he can get elected with 40-something percent of the vote by simply turning out his fact-free, zombie base. (It did not work in the House in 2018, but what else has he got?)
By contrast, most Republicans on the ballot (and others who are depending upon them to hang on so as to retain the majority) have very different goals. They want to be elected whether Trump goes down. Incumbent Republicans outside of the deepest red states need some support from Democrats and/or independents. The risk is that they will be seen as derelict in their duties, pawns for a guilty president. For them, the trial must be seen as legitimate, unless they want to risk precious votes they will need to keep their seats. (For example, Sen. Ben Sasse of Nebraska can say whatever pops into his head to justify his vote, or need not justify it at all; Sen. Cory Gardner of Colorado is going to have to figure out how not to appear like a dope for letting Trump off.) Polls consistently show about 70 percent of voters want to hear from witnesses. Refusing to allow them and conduct a real trial may offend or even anger voters.
For Republican senators not in utterly safe seats, it is far better to have something that looks like a real trial and then, by golly, explain that there just was not quite enough evidence or that — wouldn’t you know? — the trial dragged on into March so it was better to let the voters decide. If she cannot find the gumption to convict, Sen. Susan Collins (R-Maine) will at least need to be able to furrow her brow, declare what a close call this was and then side with Trump. (Just as she did in the Brett M. Kavanaugh confirmation.) Voting to acquit after a non-trial in which the president’s lawyers put on nothing approaching a serious case does not work for Collins or for other vulnerable Republicans.
Now, if you are a Democratic senator or a Democratic challenger, you want to show that the Republicans have lost moral authority to lead and will never check the president no matter what he does. They know Republicans will vote to acquit, but in doing so Democrats would love nothing more than to see Republicans look as unfair, biased, cowardly and irresponsible as possible. The next best thing to jaw-dropping testimony from former national security adviser John Bolton (or whomever) is a portrait of Republican corruption and cowardice. They may not get Bolton, but they increasingly might get a trial that turns out to be one really long ad for throwing Republicans out of power.