On the subject whether POTUS withholding military aid to Ukraine before they would investigate the DNC server in Crowdstrike's possession - quid pro quo? or no?

"18 USC 1951, the dreaded Hobbs Act, the federal anti-extortion law. Hobbs has a really nasty bite to it for public officials, because it prohibits not just bribery, but merely the collecting of political tribute under color of official right.... Also, the illegal payoff money can go to a close relative or associate, or even a charity, if there is enough of a connection to the accused, like the Clinton Foundation scam. Anybody assisting the public official in the pay-offs would also be guilty of conspiracy, at the very least."
https://www.justice.gov/jm/jm-9-131000-hobbs-act-18-usc-1951

https://www.justice.gov/jm/criminal-resource-manual-2404-hobbs-act-under-color-official-right

https://www.investors.com/politics/editorials/clinton-foundation-donations/

Democrats are trying to tie the military air to investigation of.... Biden (which was some 350 words later in the conversation with Zelensky). They are portraying this as the quid pro quo.

This is a strategic mistake on their part, because as we have a treaty with Ukrain for mutual aid in criminal investigations (as we do with many other nations), POTUS was actually fulfilling his role as Chief Executive to ensure the laws are faithfully followed/applied. Ukraine is obligated also, by treaty, to help us.

One of the ways this is being spun is by the use of "officials" as sources for articles - in this instance >>19481
(WSJ) it was a "senior DOJ official" who knew nothing of the investigation into the DNC server, which is portrayed (spun) to infer the President's CoS Mulvaney was dishonest.

Democrats will regret focusing on quid pro quo in Ukraine, because as you can see above, the illegal payoff money cannot go to an official's relative (Hunter Biden), and this will be highlighted to the fullest in any impeachment trial in the Senate.