Under the current system the Democrats only need to win in 14 states (all won by Obama in 2012) plus the District of Columbia to get to the required 270 electoral votes.
California: 55 Electoral votes (60% Obama in 2012)
New York: 29 (63% Obama)
Florida: 29 (50% Obama)
Illinois: 20 (58% Obama)
Pennsylvania: 20 (52% Obama)
Ohio: 18 (51% Obama)
Michigan: 16 (54% Obama)
New Jersey: 14 (58% Obama)
Virginia: 13 (51% Obama)
Washington: 12 (53% Obama)
Massachusetts: 11 (61% Obama)
Maryland: 10 (62% Obama)
Minnesota: 10 (53% Obama)
Wisconsin: 10 (53% Obama)
District of Columbia: 3 (91% Obama)
Total: 270 via 14 states & D.C.
to add some detail to Nero Germanicus’ post:
http://elections.gmu.edu/Turnout_2012G.html
10 million eligible didn’t show up in California in 2012; 6 million eligible didn’t show up in New York; 90 million eligible nationwide didn’t show.