A judge in Pennsylvania has dismissed a lawsuit from the Trump campaign that sought to invalidate millions of mail-in votes in the battleground state.
Judge Matthew Brann said the suit, which rested on allegations of irregularities, was "without merit".
The move paves the way for Pennsylvania to next week certify Joe Biden's win - he leads by more than 80,000 votes, reports BBC.
It is the latest blow to Donald Trump, who is trying to overturn his loss in the 3 November presidential election.
He has refused to concede and made allegations of widespread electoral fraud, without providing any evidence.
The lack of a concession has upended the process that normally follows a US election.
Mr Biden is projected to defeat President Trump 306 to 232 in the US electoral system, which determines who becomes president - far above the 270 he needs to win.
The Trump campaign has lost a slew of lawsuits contesting results from the election, and their latest efforts focus on stopping the swing states that handed Mr Biden his win certifying the results - an essential step for the Democrat to be formally declared victor.
What did the Pennsylvania judge say?
In the ruling Judge Brann wrote that the Trump campaign had tried to "disenfranchise almost seven million voters".
He said his "court has been presented with strained legal arguments without merit and speculative accusations".
"In the United States of America, this cannot justify the disenfranchisement of a single voter, let alone all the voters of its sixth most populated state," the judge wrote.
The Trump campaign argued that the state had violated the US Constitution's guarantee of equal protection under the law as some, Democratic-run counties allowed voters to fix errors on their ballots while Republican-run counties did not.
But in his ruling Judge Brann dismissed the claim, saying "like Frankenstein's Monster" it had been "haphazardly stitched together". He said even if it was the basis for a case then the Trump campaign's solution went too far.
What's the situation in other battleground states?
On Friday, Georgia dealt the Trump campaign a major blow by certifying Mr Biden's win after a by-hand recount.
The Trump campaign has two business days to request another recount.
In another swing state won by Mr Biden, Michigan, Republican officials wrote to the state's electoral board to request a two-week delay in certifying the results.
They called for an audit of the presidential vote in the largest county, home to Detroit, after it was contested by the Trump camp.
But the Michigan Department of State said delays and audits were not permitted by law.
In Wisconsin, election officials accused Trump supporters of obstructing the state's recount of votes. They said observers for Mr Trump were in some cases challenging every single ballot to deliberately slow down proceedings.
If the recount is not concluded by 1 December - the deadline for Wisconsin to certify its votes - the way is open for the Trump camp to pursue a lawsuit.