Political commentators often speak about Hillary Clinton's 'firewall,' the states she has been expected to win without much contest and which give her 272 electoral votes without having to worry about states that are currently contested.

A crack may be appearing in her firewall, however, according to Nate Silver, the founder and editor of the FiveThirtyEight political analysis site.

Silver points out that recent polls show Trump leading in New Hampshire, a state which was considered part of Clinton's firewall. While New Hampshire only has four electoral votes, losing it to Trump would force Clinton to win at least one contested state to secure the election, and if Trump wins those contested states, the electoral college vote would be tied at 269 votes apiece.

In such a situation, the state of Maine may decide the election. Like New Hampshire, Maine only has four electoral college votes. Unlike most other states, however, Maine allows for its electoral college votes to be split among several candidates. Trump has a chance to win the elector vote of Maine's Second Congressional District, which would break the tie and secure him the presidency if he gets 269 votes from all other states.

These scenarios still depend on Trump winning all contested states, so Clinton still has the advantage going into the election next Tuesday. Trump has been gaining on Clinton in the polls in recent weeks, and the effect of the FBI's reopening of the investigation into Clinton's use of a private email server as Secretary of State remains to be seen.

Trump may also benefit from a lower minority turnout, especially among the African American population, than Barack Obama enjoyed in 2008 and 2012.

Trump is trailing by 3 percentage points or less, possibly within the margin of error, in several of Clinton's other firewall states, including Colorado and Pennsylvania.