The mainstream Left has embraced Brennan and federal law enforcement bureaucrats, ostensibly because of their shared distaste for President Trump. MSNBC hired Brennan, CNN hired Clapper, and Comey / McCabe / Strzok / Page are generally treated as whistleblowers rather than leakers / criminals. Similarly, Democrats have also embraced this "Russia collusion" narrative which can be traced back conclusively to the aforementioned persons (among others). If it turns out that Brennan and other federal bureaucrats broke the law in order to jump-start an investigation connecting Trump to Russia, the Left pretty much has to own it. It will vindicate a lot of the so-called "conspiracy theories" espoused by people like Joe DiGenova, Roger Stone, etc.
If the Left doesn't want to catch political AIDS from Brennan, Clapper, Comey, the time to disassociate / disavow was last year. It's too late to backtrack and say "I personally though the Russia investigation was bogus..."