From Nate Cohn at the NYT’s Upshot blog:
In 2012, President Obama didn’t take the popular vote lead until long after he had won the Electoral College.
It led Mr. Trump to tweet that Mr. Obama had won while losing the popular vote, and say that “we should have a revolution in this country.”
A little while after that tweet, Mr. Obama took the lead, for good. He would ultimately win by almost four percentage points.
A similar pattern could unfold this year, especially with the huge Democratic margin expected in California.
Here’s a simple rule of thumb: if Mrs. Clinton is even within four or five points in the popular vote between 9 p.m. and 11 p.m., she’s probably going to win it.
If Mrs. Clinton has carried her firewall states, or either Florida or North Carolina, she could win the presidency around 11 p.m. in the East, when polls close on the West Coast.
The rest of the post is worth a read for the minutiae of the electoral map, but that’s the key bit.