The closest analog for the Democrats is Maryland. Only once has a Democrat been elected to the presidency without carrying Maryland. That was the 1948 election, in which Maryland mirrored most of the rest of the Republican northeast in backing Thomas Dewey's candidacy for the GOP. (Despite what you may have read in the newspapers, Dewey lost.)
In 2016, Maryland will be a lot easier for Hillary Clinton to win (she leads by more than 30 points, according to RealClearPolitics) than Ohio will be for Donald Trump (he trails by 3).
It also seems unlikely that Trump will reel in a number of the other top states that Republicans have relied upon in the past. New Hampshire? Maybe. Vermont? No. Illinois? Never.
For all of the time we spend considering the red and blue states in the presidential election process, it's easy to forget how recently those designations arose. Georgia is one of the states that has least consistently voted for a winning Republican -- because it, like the rest of the Deep South, was heavily Democratic until after the passage of the Civil Rights Act.
The tricky state on Hillary Clinton's list is Florida. Only twice has a Democrat won the presidency without winning Florida, in 1960 (John Kennedy) and 1992 (a guy named Bill Clinton).
In total, the winner of the presidency has won Florida in 26 of the 38 elections since 1860. That's toward the middle of the pack, as far as it goes. The states that have most often voted for the winner of the presidency are:
- Ohio, 34 of 38 times
- Illinois, 33
- California, 31
- Nevada, 31
- New Hampshire, 31
- New York, 30
- Wisconsin, 30
Clinton will certainly win at least three of those states and is currently leading in the other four.
The state that's least important for winning the presidency, regardless of party? Setting aside Alaska and Hawaii (which are still little baby states, relative to U.S. history), it's Mississippi. More than half the time, the winner of Mississippi (and Georgia, Alabama and South Dakota) hasn't won the presidency. Three of those states, Trump will win; only Georgia is a question mark.
The standard caveat applies: A pool of 38 elections isn't enough to make declarative statements about what will and won't happen. I feel comfortable saying that if Hillary Clinton loses Maryland, she won't win the presidency. If Donald Trump loses Ohio but wins Florida and Pennsylvania (for example), he could still win.
It's just never happened in 150 years.