Presidential election campaigns tend to follow a predictable issues timetable, but certain events can upset that timetable in a big way. The death of US Supreme Court justice Antonin Scalia is precisely such an event, and its consequences will be felt in November.
By the time Scalia's body reached the funeral home, US Senate Majority Leader Mitch McConnell (R-KY) had already handed Democrats a great talking point and turnout motivator with his announcement that he intends to put off Senate confirmation of any replacement for Scalia for a full year, until a new president has been elected and sworn in.
The usual tactical approach when a president of one party nominates a candidate for approval by a Senate of the other party is basically brute obstructionism -- dragging out the committee investigations, perhaps pushing back with the discovery or manufacture of scandals, and so on. McConnell could have almost certainly pulled that off. There would have been grumbling, but heck, there's always grumbling.
Alternatively, a "consensus" appointee acceptable to both sides of the aisle might be allowed to run the gauntlet. In this case, the likely pick would be DC Court of Appeals judge Srikanth Srinivasan, who clerked for "conservative" justice Sandra Day O'Connor, worked in the Solicitor General's office during the Bush administration, and was confirmed by a 97-0 Senate vote when Obama appointed him to his current post.
Instead, McConnell laid out an entirely new doctrine: When the Senate doesn't like the sitting president, he says, it will just hold off on confirming Supreme Court appointments until it gets a president it DOES like.
Why is that such a big deal? Because the implications stretch far beyond the replacement of Scalia.
At least three more SCOTUS justices -- Ruth Bader Ginsburg, Anthony Kennedy and Stephen Breyer -- are, as was Scalia, in their late 70s or early 80s. Along with Scalia, they cover the whole range from "liberal" to "conservative." And like Scalia, there's every reason to believe that they will each retire or die during the next presidential term.