This timeline suggests something I haven't seen widely reported, that the exits of the former DNI (Coats) and his top deputy (Gordon) could be intimately linked to this. But sure, it might be coincidental that Coats put in his resignation three days after the phonecall, and then Sue Gordon decided she could never report directly to Trump. One sympathizes with her, of course.

BTW, Reuters is saying that Ukraine's foreign minister denies that Trump applied any pressure on the phone call. But someone is also leaking that the phone call is only one in a series of "multiple acts" contained in the whistleblower report. When this whole story broke on Wednesday, we were explicitly told that at least some of the interaction involved an inappropriate "promise" by the president.

Do we really need evidence that Trump extorted or quid pro quo'd the Ukrainian government, or is imploring a foreign government to open up an investigation into a political opponent enough? The Republicans will absolutely refuse to impeach without explicit quid pro quo or extortion.

Edit: So I went out and checked some actual legislation pertaining to political campaigns, according to §30121. Contributions and donations by foreign nationals. After reading, I think any impeachment proceedings would have to simply argue that political intelligence against an opponent or potential opponent is a "thing of value". This seems like a straightforward argument (re: her emails), so I expect the republicans to soon begin swearing on their lives that the Biden intel would've been of no value anyway, just a total dud, nothin'. Many will successfully delude themselves. This could end with no impeachment, and handcuffs in January of 2021, without any Pence pardon. Either way, Trump will eventually argue that he just didn't know any better, which, after Bob Mueller, is going to be about the funniest thing that I'll hear in my lifetime. Oh man, what a time to be alive! Of course, I could be wrong. We'll see :).

Edit 2: I also checked out Benjamin Wittes's latest lawfareblog.org analysis, and he's got a great bit on a few avenues Congress can take to bypass the unprecedented amount of stonewalling the executive is currently conducting:

The basic problem here is that Congress is not developing the information itself. When a new revelation breaks about the president’s conduct, it breaks in the press, or in the Mueller report—or in a complaint to an inspector general. And the White House then stalls on any effort to respond to it by Congress. That stalling, it turns out, works. By the time Congress eventually holds a hearing, the public has priced the new revelation into its understanding of the president and his conduct; the revelation is not news anymore. These protracted fights give time for the impact of the revelations to be diffused. They give time for counterattacks to take hold.

Congress needs to think hard about how to raise the costs, both to the executive and to individual witnesses, of the sort of defiance it has seen—and to shorten the time frame for addressing defiance.