Trump lambasted election workers and alleged fraud in states where results from a diminishing set of uncounted votes are pushing Democrat Joe Biden closer to victory, offering no evidence.
"This is a case where they are trying to steal an election," said Trump, who spoke in the White House briefing room for about 15 minutes before leaving without questioning.
In Pennsylvania and Georgia, Biden, the former vice president, was steadily eating away the leadership of the Republican incumbent even as he maintained narrow advantages in Nevada and Arizona, moving closer to securing the 270 votes in the state-by-state Electoral College that determines the winner.
Biden, meanwhile, saw his lead from 93,000 to about 48,000 in the Arizona deal; he was ahead by only 11,000 votes in Nevada.
By winning Pennsylvania, or by winning two out of the Georgia, Nevada, and Arizona trio, Biden would become the next president. He needed to hang on to Pennsylvania and Georgia while overtaking Biden in either Nevada or Arizona. Trump's most likely path seemed narrower.
In Electoral College votes, which are largely determined by state population, most major television networks gave Biden a lead of 253 to 214 after he captured on Wednesday the crucial states of Wisconsin and Michigan.
As protesters marched for a second straight day in several U.S. cities, the election lay in the hands of civil servants who methodically counted hundreds of thousands of ballots, many of which were sent by mail in the midst of the coronavirus pandemic.
Final outcomes could take days in each state. On Thursday afternoon, Pennsylvania Secretary of State Kathy Boockvar said the state still had about 350,000 ballots to count but expected the vast majority to be counted by Friday.
In Georgia, an election official, Gabriel Sterling, said it would "take time" to process tens of thousands of remaining ballots. It was also expected that Arizona, where at least 400,000 ballots remained, and Nevada, which had 190,000 uncounted votes, would take days to complete their ballots.
Earlier in the day, Trump's remarks followed a series of Twitter posts from Trump calling for vote counting to stop, even though he is currently tracking Biden in enough states to hand the presidency to the Democrat.
Meanwhile, Trump's campaign pursued a flurry of lawsuits in Georgia, Michigan, Nevada, and Pennsylvania, though the challenges were quickly rejected by judges in Georgia and Michigan. The cases had little chance of affecting the electoral result, legal experts said.
Shortly after Trump's White House appearance, Biden wrote on Twitter, "No one will take our democracy away from us." Biden expressed confidence in earlier remarks from his hometown of Wilmington, Delaware, that he would win and urged calm as votes were counted.
"Sometimes democracy is messy," Biden said. "It also requires a little patience sometimes. But in a system of governance which has been the envy of the world, that patience has been rewarded for more than 240 years now.
A new Reuters / Ipsos poll showed a bipartisan majority of Americans rejecting the premature declaration of Trump's victory in favor of all votes being counted.
The close election emphasized the deep political divisions of the nation, while the slow count of millions of mail-in ballots served as a reminder of the deadly pandemic that continues to upend American life.
Nevertheless, if he prevails, Biden will have failed to deliver Trump the sweeping repudiation that Democrats had hoped for, reflecting the deep support that the president enjoys despite his tumultuous four years in office. Even if he ultimately loses a tight election, Trump's influence on the Republican Party will remain strong.
The winner will be faced with a pandemic that has killed over 234,000 Americans and left millions more out of work, even as the nation is still struggling with the aftermath of months of unrest over race relations and police brutality.
On Thursday night, Biden's lead in the national popular vote broke $4 million, although that plays no role in deciding the winner. In 2016, when he secured an upset victory by winning key states in the Electoral College, Trump lost the popular vote by about 3 million to Democrat Hillary Clinton.
Since fellow Republican George H.W. Bush in 1992, he has been trying to avoid becoming the first incumbent US president to lose a re-election bid.
Two Trump advisers said Trump, who often enjoyed legal battles during his turbulent business career, was at the White House working on telephones and monitoring developments on television. He spoke to state governors as well as close friends and aides and sent out some of his closest consultants to fight for him in the field.
A Trump confidant said, "He's very involved, he's monitoring, he 's talking to all of the states." This guy doesn't look good, but he wants to continue fighting. Right now, he's in a fighting mood. He's not dejected or melancholy. But the path is becoming more and more difficult.
Ever since Election Day, Twitter and Facebook have flagged numerous Trump posts as misleading.
However, Trump's rhetoric gained traction with some supporters. On Thursday, before the social media giant took down the page, a Facebook group called "Stop the Steal" pushing false claims of voter fraud gained hundreds of thousands of members, citing calls for violence.
Small protests outside voting centers were also held by supporters of both candidates on Thursday, though the demonstrations were largely peaceful.