Blackmail...

Barr is correct on both counts, there was snooping on the Trump campaign, and  an ongoing intelligence investigation became public.

The abuse of power here is what happened after Trump won the election. This is when the investigations themselves and other kinds of surveillance were disclosed to the press. Details about incoming National Security Adviser Michael Flynn’s conversations with the Russian ambassador during the presidential transition leaked. So did the FBI’s probe of Trump associates and their contacts with Russia, along with the existence of the surveillance warrant on Page himself.  

The New York Times reported in early 2017 that, in the days and weeks leading up to Trump’s inauguration, intelligence and White House officials distributed intelligence on contacts between Russia and Trump associates far and wide.


Brennan briefed was then-Senate Minority Leader Harry Reid, who a few days later released a letter to then-FBI director James Comey, urging him to make public intelligence about the Trump campaign and Russia. 

After the election, Brennan became a TV pundit and used his platform to accuse Trump of treason and claim the Russian government was blackmailing him. 
[Eil Lake, Bloomberg]