WASHINGTON (REUTERS) - Despite an emphatic court setback in Pennsylvania, US President Donald Trump showed no sign on Sunday (Nov 22) of dropping his long-shot efforts to overturn the US presidential election even as Democratic President-elect Joe Biden moves ahead with planning for his incoming administration.

Since Mr Biden was declared the winner of the Nov 3 election two weeks ago, the Republican president has launched a barrage of lawsuits and mounted a pressure campaign to prevent states from certifying their vote totals.

Critics of Mr Trump, including Democrats and some Republicans, have accused him of trying to undermine faith in the American electoral system and delegitimise Mr Biden's victory by promoting false claims of widespread voter fraud. Mr Biden is due to take office on Jan 20.

"Fight hard Republicans," Mr Trump wrote on Twitter on Sunday morning as he pressed his unsubstantiated narrative of voter fraud.

So far, attempts to thwart certification of vote tallies have failed in courts in Georgia, Michigan and Arizona.

On Saturday, US District Judge Matthew Brann, a Republican appointed by Democratic former President Barack Obama, dismissed a similar effort in Pennsylvania, writing that the case presented by Mr Trump's lawyers claiming voter fraud amounted to"strained legal arguments without merit and speculative accusations".

"This claim, like Frankenstein's Monster, has been haphazardly stitched together," Mr Brann wrote.

Some of Mr Trump's fellow Republicans in Congress are now breaking ranks though many, including the most senior ones, have not.

Republican Senator Pat Toomey said Mr Brann's ruling "exhausted all plausible legal options" for Mr Trump in Pennsylvania, and he urged the president to "accept the outcome of the election".

Mr Toomey also congratulated Mr Biden and Vice-President-elect Kamala Harris and referred to them as "dedicated public servants".

Ms Liz Cheney, a member of the Republican leadership team in the House of Representatives, earlier called on Mr Trump to immediately present evidence of widespread voter fraud or otherwise respect "the sanctity of our electoral process".

Mr Ron Klain, Mr Biden's choice as incoming White House chief of staff, and other Biden aides were due to appear on Sunday morning news shows to give their team's perspective on the transition process.

For Mr Trump to have any hope of remaining in the White House, he needed to somehow overcome Mr Biden's 81,000-vote margin in Pennsylvania, a state he won in 2016. The state is due to begin certifying its results on Monday.

Mr Trump's lawyers vowed a quick appeal, but lawyers who opposed him in court said he is out of time.

"This should put the nail in the coffin on any further attempts by President Trump to use the federal courts to rewrite the outcome of the 2020 election," said Ms Kristen Clarke, president of the Lawyers' Committee for Civil Rights Under Law.

Trump refuses to concede

Mr Biden received 6 million more votes nationwide than Mr Trump and, more importantly, prevailed 306-232 in the state-by-state Electoral College system that determines the election's victor.

Mr Biden has spent the past weeks preparing to take office and choosing members of his administration, though Mr Trump's administration has refused to provide the customary funding and security clearances for incoming presidents.

Critics have said Mr Trump's refusal to concede carries serious implications for national security continuity and in impeding the fight against the coronavirus pandemic that has killed about 256,000 Americans.

In order to remain in office, Mr Trump would somehow need to overturn election results in at least three large states - an unprecedented feat in US history.

A recount in Georgia only affirmed Mr Biden's victory there - a state Mr Trump had won in 2016 - and officials certified the result on Friday. Mr Trump's campaign said late on Saturday it would request another recount.

In Wisconsin, election officials have criticised Trump volunteers for slowing a partial recount that is not expected to overturn Mr Biden's victory. Mr Trump also won Wisconsin in 2016.

With recounts and lawsuits coming up short, Mr Trump is now pressuring Republican-led state legislatures to throw out the vote totals and declare him the winner.

"Hopefully the Courts and/or Legislatures will have ... the COURAGE to do what has to be done to maintain the integrity of our Elections and the United States of America itself," he wrote on Twitter after the Pennsylvania ruling.

On Friday, he summoned two top Republicans in Michigan's legislature to the White House. After the meeting, they said they saw no evidence that would lead them to intervene. Biden is leading Trump in Michigan by 154,000 votes.

Election officials across the country say there is no evidence of significant voting fraud, and Trump's own administration has called the election the "most secure in American history".

But Mr Trump's accusations have continued to inflame his hard-core Republican base. Half of Republicans believe the election was stolen from Mr Trump, according to Reuters/Ipsos polling, and some supporters have staged rallies across the country to protest the outcome.