Critical Counties is a CNN series exploring 11 counties around the country that chief national correspondent John King has identified as key in the 2016 election. These counties could play an outsize role in the election and offer an instructive example of what the political parties have accomplished. Bucks County is the 11th and final county in the series. Explore the other critical counties.

On election night, watch to see if Hillary Clinton's firewall materializes.

It's an ambiguous, ill-defined idea, but the gist is that if Clinton can lock up a few choice states, she will win the presidency. If Donald Trump can bust through, his path to victory remains open.

One way or another, the firewall can't happen without a Democratic victory in Pennsylvania. History is on Clinton's side, as Democrats have won this state every election since Bill Clinton in 1992.

To see if Trump could upset that trend, look to Bucks County.

Situated outside of heavily Democratic Philadelphia, this area could be more open to the Republican nominee. If voters here turn out in his favor, it could make room for a Trump win in Pennsylvania.

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