US President Donald Trump's campaign has filed a lawsuit in a Pennsylvania federal court, seeking an emergency injunction to stop state officials certifying Joe Biden's victory in the state.
The lawsuit alleges Pennsylvania's mail-in voting system "lacked all of the hallmarks of transparency and verifiability that were present for in-person voters".
"Voters in Pennsylvania were held to different standards simply based on how they chose to cast their ballot, and we believe this two-tiered election system resulted in potentially fraudulent votes being counted without proper verification or oversight," Matt Morgan, Trump 2020 general counsel, said on Monday.
The lawsuit was filed against Pennsylvania Secretary of State Kathy Boockvar.
Her office did not immediately respond to a request for comment.
The president, who has spent months trying to undermine the election results with unproven allegations of fraud, has pledged to go forward with a legal strategy he hopes will overturn state results that gave Biden the win in Tuesday's vote.
The Trump campaign and Republicans have brought numerous lawsuits over alleged election irregularities.
Judges have already tossed cases in Georgia and Michigan.