Bribery?
I would want to see Democrats who favor impeachment to make their case as Constitutionally-grounded as possible, but the obvious question is whether there's a credible case to be made that the President might have engaged in treason, bribery, and other high crimes and misdemeanors.
The federal bribery statute at 18 U.S.C. §201(b)(2) "requires a showing that something of value was...corruptly demanded, sought, received, accepted, or agreed to be received or accepted by a public official...in return for 'being influenced in the performance of any official act'" according to the Supreme Court in
United States v. Sun-Diamond Growers of California, 526 U.S. 398, pp. 404-405 (1999).
When Zelenskyy brought up the subject of American support against Russian invasion, Trump said he needed a favor though. The two favors Trump asked for were indeed something of value, and they both inure to the benefit of the Trump Campaign rather than U.S. national security interests, thereby meeting the element of corrupt intent.
I do not mean to imply that Supreme Court interpretation of federal statute law provides us with a controlling interpretation of more foundational constitutional law—especially when dealing with this new and heavily originalist iteration of the high court—it is possible the founders had a narrower conception of bribery in mind. As to that, we'd best consult Blackstone:
Blackstone said:
Bribery is the next species of offence against public justice, which is when a judge, or other person concerned in the administration of justice, takes any undue reward to influence his behavior in his office.
The undue reward here consists of the two favors Trump specifically asked Zelenskyy to come up with, the first designed to clear the Trump 2016 campaign of wrongdoing and the second designed to help give a boost to Trump 2020.
I've no idea how the GOP folks are going to work around this one, but I'm guessing they are going to focus on the requirement of corrupt intent. They will perhaps come up with plausible sounding reasons to be chasing down a
bizarre conspiracy theory about a DNC server which was somehow exfiltrated from the United States.
