WASHINGTON (BLOOMBERG) - US President Donald Trump's legal campaign to overturn his 2020 election defeat shrank further with the narrowing of a key Pennsylvania lawsuit and the retreat of supporters who had sued in four battleground states.

The Trump campaign on Sunday (Nov 15) revised its highly touted suit to block certification of President-elect Joe Biden's projected Pennsylvania victory, abandoning its most extravagant claims of widespread voter fraud.

On Monday, four suits associated with a conservative group that also sought to block election certifications based on broad voter-fraud claims were dropped in Pennsylvania, Michigan, Wisconsin and Georgia.

The broad pullback follows several major courtroom defeats last week and highlights the difficulty lawyers are having in backing up Mr Trump's claims of a "rigged" election.

"It's an understatement to call the remaining claims weak; they're absurd," said Wendy Weiser, who heads a democracy program at New York University's Brennan Center for Justice and isn't involved in the case.

"They're not even purporting to base any claims on the existence of voter fraud because they haven't been able to find any evidence to suggest that is even possible."

Even if it succeeds, the revised Pennsylvania suit, which faces a hearing on Tuesday on the state's motion to dismiss, may not challenge enough ballots to flip a state that Mr Biden is currently carrying by more than 68,000 votes.

The Trump campaign had previously focused its claims on the idea that as many as 680,000 mail-in ballots cast in the Democratic strongholds of Philadelphia and Pittsburgh should be invalidated because Republican observers couldn't adequately monitor the vote-counting for fraud.

The suit now focuses on a narrower category of allegedly "defective" ballots that some voters were allowed to remedy.

A number of suits making similar claims were dismissed on Friday by county courts in Philadelphia and its suburbs, with judges saying the Trump campaign and its allies were baselessly asserting errors where none existed. The state has said there were not enough ballots at issue in those suits to change the outcome of the race.

The retreat elicited scorn on Monday from lawyers for the Democratic National Committee, which called the revision "an implicit admission that they lack evidence to support their claims."

The Trump campaign said on Monday it stands by its assertion that ballots were improperly counted without observers present, even though the allegation is no longer part of specific claims in the complaint.

"We are still arguing that 682,479 ballots were counted illegally, in secret," Tim Murtaugh, the campaign's communications director, said in a statement on Monday. "Our poll watchers were denied meaningful access to watch the vote counting and we still incorporate that claim in our complaint."

The Trump campaign still has a federal suit seeking to block certification of the election results in Michigan based on broad fraud allegations. It has submitted dozens of affidavits from Republican poll watchers who say they witnessed suspicious activity during the vote count at a Detroit convention centre.

But a similar state court suit by allies of the Trump campaign was rejected on Friday by a judge who found the affidavits of poll watchers to be "not credible" and "rife with speculation and guesswork about sinister motives."

Like the Trump campaign's original Pennsylvania complaint, four lawsuits filed last week by plaintiffs associated with conservative group True the Vote claimed widespread fraud in Democratic-leaning counties. The suits in effect asked federal courts to block certification of election results in Michigan, Wisconsin, Georgia and Pennsylvania unless all votes cast in those states' largest cities and counties were invalidated.

True the Vote had promised to put forth evidence including "expert reports based on data analysis" comparing mail-in ballots with state voter-registration databases, US Postal Service records, Social Security records, criminal records, motor-vehicle records and other sources "by using sophisticated and groundbreaking programs to determine the extent of illegal voters and illegal votes." No reason was given for the suits' abrupt withdrawal on Monday.

Jim Bopp, a lawyer for True the Vote, declined to comment.

The True the Vote suits were the only legal challenges to the vote counts in Georgia and Wisconsin, and the Trump campaign dropped its suit in Arizona on Friday. That means the campaign currently has no path to overturning Biden's victory in the courtroom, since Trump needs to move at least three states into his column.

Even if he managed to succeed in both the Michigan and Pennsylvania lawsuits, he still wouldn't have enough electoral votes to retain the White House. The president's lawyer Rudy Giuliani has more suits will be filed.

And both of those cases stand a good chance of being dismissed quite soon. Pennsylvania Secretary of State Kathy Boockvar filed a fresh motion to dismiss shortly after the Trump campaign revised its complaint, taking on its claim that voters were harmed by the fact that Democratic-leaning counties had different procedures for allowing voters to fix ballot errors.

"Plaintiffs fail to identify how one county's offering an opportunity for a voter to cure his or her defective ballot before the close of the polls could possibly burden the rights of a voter in a separate county," Boockvar said.