It is a big deal.
Since it seems he got a FISA court to give him a warrant on the second try, I’m not sure if it’s illegal, however the big deal is that it might expose the leakers of classified material within the gov’t. which is highly illegal.
For Obama, the appearance of wiretapping a political opponent that he campaigned against smells like Nixon and Watergate.
Trump needs to purge all Obama appointees immediately and go on complete offense and have Sessions investigate everything including Pizzagate.
As someone pointed out, this is Civil War 21st Century style.