The thing is, we don't need evidence. Trump has admitted what he did, multiple times, on camera.
....the story behind the impeachment case is relatively simple: Congress approved military aid for Ukraine, but Trump withheld it as part of a sustained campaign to pressure Ukraine into launching an investigation of his political rival Joe Biden’s family. There’s a record of him doing it. There are multiple credible witnesses to the phone call and larger campaign. Several Trump allies and administration officials have admitted to it on camera. Trump himself admitted to it on the White House lawn. (Source)
Why would he do this?
Because he thinks he's above the law. Like the spoiled child he has always been, he believes there is one law for him, and another for all "those people." He sincerely believes he can do whatever he wants.
It doesn't help, of course, that Fox News, his main source of information these days, confirms this worldview 24/7.
These impeachment hearings are about whether he's right. Do we have a country that is based on laws, or do we have a country where men like Trump, and Kavanaugh, and Epstein, Jeff Bezos and Clint Lorance, can do whatever they want, and get away with it?
I know the world Fox News wants. What world do the rest of us want?