Trump signs autographs after a rally in Harrisburg, Pennsylvania, April 21, 2016. Photo credit: Mark Makela / Getty

One of the most pervasive myths in American politics is that a “Big Blue Wall" will protect Democratic presidential nominees, perhaps even those who lose the popular vote. In truth, this electoral Blue Wall is more like a collection of disconnected forts—some imposing, some not—and the loss of any one of them would likely doom the Democratic nominee.
The Blue Wall—states where the Democrats have won every presidential race since 1992—includes the entire Northeast except for New Hampshire, the Midwestern Great Lakes states aside from Ohio and Indiana, the three Pacific Coast states, and Hawaii. Even holding the wall is no guarantee of victory. John Kerry won the entire Blue Wall in 2004, plus New Hampshire, yet still lost to George W. Bush by 35 electoral votes. Al Gore won the entire Blue Wall in 2000—back when it was worth 13 more electoral votes—plus Iowa and New Mexico, yet still lost to Bush by 5 electoral votes. For the Democrats, holding the Blue Wall is necessary but not sufficient.
It is often claimed that Donald Trump has several must-win states, and this is true (although the states listed are often wrong). In addition to the 23 states that Mitt Romney won by at least 7 percentage points, which Trump isn't going to lose, Trump has 3 must-win states: Ohio, Florida, and North Carolina. (Polling finds that Trump narrowly overtook Clinton in mid-September in all three.) If Hillary Clinton wins any of those states, it'll be a knockout blow.
A Trump Road to 270 Could Go Through Wisconsin
Bloomberg
But in addition to the 15 states where President Obama beat Romney by at least 10 points, which Clinton isn't going to lose, Clinton has 5 must-win states: Pennsylvania, Michigan, Wisconsin, Minnesota, and Virginia. (Polling finds her ahead in all five.) The first four are part of the Blue Wall; the Old Dominion is not. If Trump wins any of those states, he'll be headed to the White House.
So there are eight potential knockout states in this election, and five of them are being defended by Hillary Clinton.
If both candidates win their respective must-win states (and split the two competitive congressional districts in Nebraska and Maine), that will bring the electoral-vote tally to 260 for Clinton and 253 for Trump, with Colorado, Nevada, Iowa, and New Hampshire (25 combined electoral votes) still in play. Clinton's task would then be to win any two of the four remaining states; Trump's task would be to win any three. If Clinton wins Colorado and Trump wins the other three, the candidates would be tied at 269, with the election moving to the House of Representatives.
The Five Paths
Clinton's path to 270 is relatively simple: (a) steal Ohio, Florida, or North Carolina; or (b) win her five must-win states and prevent Trump from getting 16 more electoral votes from among the 25 remaining on the board. (If Clinton wins right-leaning Ohio, Florida, or North Carolina, there's no way she'll fail to win the requisite number of left-leaning states.) Trump's task is to win his 3 must-win states (plus the 23 states that are givens) and then follow any one of five different paths to winning the remaining 17 electoral votes he'd need to achieve victory.