PHILADELPHIA (BLOOMBERG) - An amended filing by United States President Donald Trump's campaign on Sunday (Nov 15) significantly narrowed the scope of an effort that had earlier sought to scrap about 680,000 votes cast in Pennsylvania.

The lawsuit originally sought to block Pennsylvania from certifying its election results unless it threw out mail-in votes from the state's most-populous counties, which include the Philadelphia and Pittsburgh areas that voted heavily in favour of Democrat Joe Biden.

It alleged that Trump-affiliated observers were limited from watching votes being counted in Democratic strongholds.

Sunday's updated filing scrapped that argument, to instead focus on ballots it maintains were improperly "cured" - an unknown number but one that is almost certainly a fraction of Mr Biden's lead in the state of almost 66,000 votes.

"There is no doubt the Trump lawsuit in PA suffered shrinkage today," Democratic lawyer Marc Elias said on Twitter.

The campaign's original filing in federal court on Sunday claimed there's evidence that voters in Democratic-leaning counties were improperly allowed to fix errors, or cure, their ballots.

A half-dozen counties named in the suit illegally allowed voters who cast deficient ballots to cast replacement absentee and mail-in ballots before Election Day or provisional ballots on Nov 3 to "cure" any issues, Mr Trump's campaign said.

Attorneys quit

"Unless Bush v Gore was much ado about nothing, presidential candidates of course have an interest in having lawful votes for them counted and unlawful votes for their opponent invalidated," the filing said.

"That's particularly true in Pennsylvania, one of a few swing states where recounts or other litigation is ongoing and where the vote margin is close."

The filing was made by campaign lawyer Linda Kerns, two days after attorneys with Porter Wright Morris & Arthur LLP walked away from the case.

The campaign said that firm "buckled" under criticism from "leftist mobs". A hearing on the state's motion to dismiss the lawsuit is set for Tuesday, with a separate evidentiary hearing scheduled for two days later. The case will be decided by US District Judge Matthew Brann in Williamsport, Pennsylvania.

The suit is part of Mr Trump's last-ditch effort to reverse the result of the election, which he continues to claim without evidence was "rigged" against him by Democrats.

A suit seeking to block Michigan from certifying its result for Mr Biden is also pending. Legal experts say the cases are likely designed to reinforce a political strategy to undermine Mr Biden rather than win in court.

Pennsylvania Attorney-General Josh Shapiro's office didn't immediately return a message seeking comment.

'Our big cases'

Mr Trump on Sunday evening suggested more legal action could be ahead, tweeting that "our big cases" will be filed soon, as states push up against deadlines to certify the votes in the Nov 3 election. For Pennsylvania, that date is Nov 23.

The campaign's initial filing on Sunday cited Pennsylvania's Lancaster County, which Mr Trump won, as an example of how things were done right, because election officials there "did not contact voters who submitted defective ballots or give them an opportunity to cure", the filing said.

"They simply followed the law and treated these ballots as invalid and refused to count them."

Another plaintiff in the case, voter John Henry, alleges his defective vote was treated differently from those of voters in other counties in violation of the Constitution's Equal Protection Clause.

"In other words, Henry cast a defective ballot that was not counted, but another Pennsylvania voter in another county could cast the same defective ballot and have his vote counted - solely based on place of residence," the campaign said.

"The Defendant counties' insistence upon counting illegal ballots disparately favoured Democratic-leaning counties over Republican-leaning counties."

With almost all ballots counted in Pennsylvania, Mr Biden is ahead by almost one full percentage point.