EXPLAINER: Why AP hasn't called Pennsylvania

WHY AP HASN'T CALLED PENNSYLVANIA:

A close margin and a large number of outstanding votes are what's making the Pennsylvania contest between President Donald Trump and Joe Biden too early to call.

The Democrat held a lead over Trump of more than 28,800 votes at 11:15 p.m. EST Friday, out of more than 6.5 million ballots cast — a edge of about 0.43%. State law dictates that a recount must be held if the margin between the two candidates is less than 0.5%.

The Pennsylvania secretary of state's website said Friday that there were about 89,000 more mail ballots to count. Many were from Allegheny County, a largely Democratic area that is home to Pittsburgh, and the Democratic stronghold of Philadelphia County.

Additionally, there are potentially tens of thousands of provisional ballots that remain to be tabulated, though an exact number remained unclear. Those ballots will be counted after officials verify their eligibility to be included.

Pennsylvania is among a handful of battleground states Trump and Biden are narrowly contesting as they seek the 270 electoral votes needed to win the presidency.

Trump, who held a 675,000-vote lead early Wednesday, prematurely declared victory in the state.

The late-counted ballots were overwhelmingly in Biden's favor.

One reason the race tightened: Under state law, elections officials are not allowed to process mail-in ballots until Election Day. It's a form of voting that has skewed heavily in Biden's favor after Trump spent months claiming — without proof — that voting by mail would lead to widespread voter fraud.

There's a possibility the race won't be decided for days. If there is less than a half percentage point difference between Biden and Trump's vote total, state law dictates that a recount must be held.

Democrats had long considered Pennsylvania a part of their "blue wall" — a trifecta that also includes Wisconsin and Michigan — that for years had served as a bulwark in presidential elections. In 2016, Trump won each by less than a percentage point.