The news also broke yesterday that George Papadopoulos, another Trump campaign official, had earlier pleaded guilty to one criminal charge and is now cooperating with prosecutors. Papadopoulos was prosecuted for lying to FBI agents about his role in colluding with Russian agents to obtain Hillary Clinton's hacked emails.
While the Manafort and Gates indictments and arrests grabbed the majority of media attention, the more damning reveal is the Papadopoulos news, which should terrify Trump. His former campaign worked is spill all to prosecutors, revealing everything he knew about that went on inside the campaign.
This is like Watergate deja vu. Trump repeatedly denied his campaign colluded with the Russians, and Mueller has now destroyed that claim.
Lawfare published a great overview on the big news yesterday (link here).
They make the case concisely: Trump's campaign chair was an unregistered foreign agent of a Putin puppet government and Trump's campaign was colluding with Russian emails to get stolen emails. Trump repeatedly lied about no Russian collusion and it has now been repeatedly documented as having occurred multiple times.
Read the Lawfare piece. It nicely establishes why the news yesterday was just about as bad for Trump as it could possibly be.