The Philadelphia Convention Center will become the center of election activity in the city on Tuesday at 7 a.m., as hundreds of election officials start working to process hundreds of thousands of mail-in ballots.

Their work in Pennsylvania's largest city -- a key Democratic stronghold -- could be crucial to determining who wins the battleground state next week, and with it, the White House.

Under pressure to work quickly, officials are also being squeezed by the calendar, after Republicans in control of the state legislature opted not to allow counties to begin processing mail-in ballots before the election, as is the case in most other states.

That abridged timeline, the potential for legal challenges and the sheer volume of mail in ballots in Pennsylvania -- which is allowing voting by mail for the first time this year -- could complicate a ballot count that is expected to continue until Friday, according to the state's top election official.

-ABC News' Benjamin Siegel

On ABC's "This Week," Chief Justice Correspondent Pierre Thomas talked about law enforcement's greatest fear on Election Day.

“Law enforcement is feeling the stress as we race towards Tuesday … there’s a real worry about white supremacists, anti-government extremists and anarchists, who might see such a toxic environment as an opportunity for violence.”

Counting the votes “might take some time in this election … and everyone watching at home should know that it is not a sign that anything has gone wrong,” said ABC News Chief Anchor George Stephanopoulos on "This Week" Sunday.