Biden is opening up a big lead over Trump in the Philly suburbs as the ‘blue shift’ takes hold in Pa.

Joe Biden’s lead in the Philadelphia suburbs is growing by the minute as elections officials continue to count scores of mail ballots, and his strong performance there may well help lift him to capture Pennsylvania’s 20 Electoral College votes.

Biden is on track to beat President Donald Trump in Philadelphia’s four collar counties by a bigger margin than Hillary Clinton did in 2016, if current trends hold with regard to the candidates' share of mail ballots. Biden currently has 58% of the more than 1.4 million votes counted in those counties — a 17-point edge over Trump’s 41%. Clinton won those counties by 14 percentage points.

In Montgomery County, where officials said Thursday they’d tallied all ballots received by 8 p.m. on Election Day, Biden leads with about 63% of the vote, over Trump’s 36%. Biden’s 131,000-vote edge over Trump exceeds Clinton’s 2016 advantage there by about 38,000 votes.

Biden leads by about 3,000 votes in Bucks County, but if current trends hold, that will only grow as outstanding mail ballots are counted. And Biden is winning by big margins in Chester and Delaware Counties.

Biden’s growing advantage in the suburbs reflects a phenomenon known as “the blue shift.”

On Tuesday night, and into early Wednesday morning, preliminary election results showed Trump narrowly ahead in Chester County, which Clinton won by almost 10 percentage points.

But just 38% of some 146,000 mail ballots received by officials there had been counted.

By Thursday morning, Biden had pulled ahead with about 177,000 votes, compared to Trump’s 126,000 — a 17 percentage point advantage.

The number of mail ballots to count is expected to increase. Some ballots had been received but not yet scanned into the state’s database. The Pennsylvania Supreme Court also allowed mail ballots to be counted if they are returned to county elections offices by 5 p.m. on Friday, instead of the normal deadline of 8 p.m. on Election Day. Republicans are challenging that ruling before the U.S. Supreme Court.

So as elections officials continue to count mail ballots, it’s likely that Biden’s lead in the suburbs will grow even more. He continued to face a significant deficit statewide on Wednesday, but with more than a million mail ballots to count.

“It is very much a waiting game. We’ve all gotta be patient,” Dick Bingham, chairman of the Chester County Democratic Committee, said Wednesday. “It’s nerve-wracking. But we knew it was going to be close.”

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Republicans said Trump was beating expectations in the suburbs.

“He’s doing better in some of these places than I thought he would, and that’s not a party position,” said Charlie Gerow, a Pennsylvania GOP strategist.

Even in heavily Democratic Delaware County, Gerow said if Trump can prevent a bloodbath, it could mean he hangs on to the state.

“If the president is above 40% in Delaware County he, in my judgment, most certainly wins Pennsylvania," Gerow said. As of Thursday morning, Trump had 38% of the vote in the county.

Gene DiGirolamo, a Republican Bucks County commissioner, said that “a lot of people thought Biden would win Bucks County by double digits.”

“That’s certainly not the case,” he said.

This kind of “blue shift” was expected: that in-person votes would be counted first, and favor Trump, but that mail ballots would favor Biden, and that his numbers would climb faster as that count continued.

State law required officials to wait until Election Day to start processing mail ballots. It was long expected that it would take several days to count them all.

-Staff writers Julia Terruso and Jonathan Lai contributed to this article.