First of all, if you want (or need) a good summary of the current state of the Trump-Ukraine situation, Electoral Vote has a good one today that can bring you up to speed.
Of course, things are moving quickly so that article is already out of date. In particular, Nancy Pelosi is about to announce a formal impeachment inquiry into Donald Trump.
And you know what? I totally agree. I think it is time to impeach. What convinced me was this article by Zack Beauchamp in Vox. It is a good article and you should read it, but the main point is this:
Impeaching Trump over Robert Mueller’s findings in the Russia investigation would have been an attempt to address past offenses; impeaching Trump over these calls would be an attempt to halt what sure looks like an ongoing attempt to hijack American foreign policy in service of the president’s reelection. Democrats have an obligation to try to stop this before it gets any further.
Trump believes he can get away with anything (including shooting someone in downtown NYC). If we don’t stand up to him when we catch him doing something, (as David Frum points out) he will just get worse and worse. He thinks he can sweep this scandal under the table like he has before, but most of those scandals were in the past. He is doing this one now, and we have an obligation to stop him.
Even if the Senate doesn’t convict him. At the very least, it will put all the spineless Republican enablers of Trump into the awkward position of saying that the president of the US is above the law.