Hillary must be concerned and worried about the vote in Pennsylvania because she and her surrogates have been campaigning there, and in Pittsburgh specifically, every day.

Yesterday, Hillary was in Pittsburgh.  Last week Bon Jovi was at the University of Pittsburgh trying to rally the students.  Two weeks ago, Hillary and Kaine were in Pittsburgh's 14th Ward, a predominantly very liberal section home to many college professors, doctors, lawyers, and historically the "Jewish" area of Pittsburgh.  Hillary is trying to rally her base.  Hillary is up an average 3 points, but she is spending lots of time and money in Pennsylvania.  The Senate race between Republican Pat Toomey and Democrat Katie McGinty shows McGinty with a slight lead.  Toomey is another weak Republican, like Paul Ryan, who has not endorsed or supported Trump.

As reported on 08/24/2016:

Pennsylvania's Democratic registration edge has shrunk from 1,135,173 to 917,192 since November 2012, but most of that is attributable to party switching[.]

But this voter registration lead is misleading.  In Pennsylvania's primary system, you can vote only for the party for which you are registered.  This causes many who live in the cities to register as Democrats so they can vote in the primary for mayor and council, because the real election is the primary.  For example, Pittsburgh has had Democratic mayors for the last one hundred years.  The primary elections are usually a choice among an Irish Democrat, a Jewish Democrat, or an Italian Democrat, and sometimes a combination thereof.  Many of the mayors have been social and economic conservatives who would be more at home with the national Republican Party instead of today's Republican Party.

The problem for Republicans is Philadelphia.  It usually produces a 300,000-vote advantage for the Democrats that must be overcome by what is referred to as the "T," which means the northern and central counties that vote overwhelmingly Republican.

But Hillary's attention to Pittsburgh means that her internal polls show weakness.  At this stage, Pennsylvania should be safe for her, and she should be in Florida and North Carolina.  Yet she, and her surrogates, are in Pennsylvania almost daily.  Pittsburgh seems safe but not overwhelming for Hillary, but the counties in southwestern Pennsylvania seem strong for Trump.  The small towns outside Pittsburgh have been devastated by the closing of mills and factories, which makes them responsive to Trump's attack on NAFTA and free trade.  The city of Pittsburgh has recovered to some extent from the closing of the steel mills because of the creation of jobs in the hospitals and universities.  Pennsylvania is also a strong Second Amendment state.

The key is how many votes Hillary gets in Philadelphia.  Hillary has scheduled an event in Philadelphia on election eve, Monday, with Barack and Michelle Obama, and a Saturday, November 5 event with Katy Perry in Philadelphia.

It appears from Hillary's attention to Pittsburgh, and the rest of Pennsylvania, that Hillary is worried.  If Trump wins Pennsylvania, he wins the election.