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Joe Biden was declared the winner of the 2020 presidential election after the Associated Press and other outlets declared him the winner of Pennsylvania. Although the vote margins are close in Wisconsin, Arizona and Georgia, Pennsylvania was the state that tipped the scale enough to call the election.

So, how did he win Pennsylvania? The New York Times attributed Biden’s win to “counties east of the Appalachians [that] shifted left.” The Washington Post argued that “it wasn’t Pennsylvania’s major urban centers that set the result in 2020.” Instead, they wrote, “It was Erie County and other places like it, where relatively minor shifts across a wide swath of small, industrial cities, growing suburbs and sprawling exurbs.”

But if Allegheny County voted for Biden as predictably as it had for Democratic candidates the past five elections, the results this year could still be uncertain. It was Biden’s unusual, historic performance in Allegheny County, alongside one suburban Philadelphia county, Montgomery County, which provided enough of a margin for Biden to definitively win.

As of Wednesday evening, Allegheny County had already recorded the vast majority of its votes, more than 717,000, the largest number of ballots cast since more than 719,000 votes were cast when Lyndon Johnson was elected in 1964. And the 1964 election was near the peak of the county’s population boom when about 30% more people called Allegheny County home. There are still some provisional and overseas ballots that haven’t yet been included and just under 1,000 additional ballots that were postmarked by Election Day and arrived within three days.

Five takeaways:

  1. Allegheny County was one of two counties in Pennsylvania, along with Montgomery County, where Democratic votes increased enough to give Biden a definitive win.
  2. Both presidential candidates increased the number of votes their party received compared to 2016.
  3. Biden is winning by nearly 146,000 votes, the biggest margin in Allegheny County since 1964. The urban core and most of the suburbs voted for Biden. Trump’s wins came largely on the edges of the county such as in Findlay, Fawn and Elizabeth townships.
  4. Some of the biggest gains for Biden from 2016 were in suburban and rural precincts, some of which he still lost. Some of Trump’s improvements were in primarily Black neighborhoods in the urban core.
  5. While Trump expanded to the urban core, Biden expanded there too, and almost everywhere else,  ultimately winning the county by the largest percentage since 1992.

Allegheny County was one of the two most important counties for Biden

As of Wednesday night, Biden led Donald Trump by 51,301 votes in Pennsylvania, according to the state tally, enough votes to prevent an automatic recount and likely enough votes to survive any legal challenges that Trump attempts.

Allegheny County delivered around 47,000 more votes for Biden than the average Democratic presidential candidates since 2000. And he won more than 37,000 additional votes compared to Hillary Clinton in 2016. Only Montgomery County moved in Biden’s direction by about the same number of raw votes: he’s winning Montgomery by about 37,000 more votes than Clinton did according to the state tally. Allegheny and Montgomery Counties are the second and third largest counties in the state, less populous than Philadelphia.

The last five presidential elections in Allegheny County have had predictable results. The Democratic candidate earned between 56-57% of the vote, the Republican 40-42%. The Democratic candidate won by between 90,000 and 108,000 votes. The average number of votes Democrats gained in those five elections was 98,236.

That predictability changed dramatically this year: Allegheny County has already delivered at least 145,815 more votes to Biden than to Trump.

If Biden had performed just as well as Clinton in Allegheny County — with all else being equal —  he would be winning the race in Pennsylvania by fewer than 15,000 votes. That likely would have triggered an automatic recount and could have made Biden’s margin more susceptible to legal challenges.