Mother Jones‘ Kevin Drum sez it plainly::
Trump used military aid as a way to extort a personal benefit from Ukraine. That’s the start and the finish of the whole thing. If he had used military aid as bait to get Ukraine to fall in line with American interests in one way or another, no one would have blinked.
But he didn’t. He used the aid as bait to get a personal favor from Ukraine: namely an investigation into an opponent in a presidential campaign. That’s as plain an example of abuse of power as you could imagine. And after he was caught, Trump didn’t apologize or promise not to do it again or anything like that. He conducted a scorched-earth attack that left Democrats with no choice but to impeach.
The Senate Republicans all know this. It’s not complicated. They simply believe it’s more important to cover for this president than to acknowledge what he did and hold him accountable.
They are just like him. They’ll keep this president in office, and themselves, by any means necessary. There are no limits.