Democrat Joe Biden is closing on President Donald Trump in tense vote counts in Pennsylvania and Georgia, two states that the current occupant of the White House cannot afford to lose if he is to keep alive his hopes of a second term.

Biden relentlessly piled up votes through the night and all day as counting goes on in the two states, racking up huge margins among mail-in ballots favored by Democratic voters.

As the drama unfolded across the country, the President's allies launched legal challenges and floated conspiracy theories as Trump tweeted "Stop the Count!"

Tight races are also going into overtime in two other states that could influence the destiny of the race, Nevada and Arizona.

Biden so far has 253 electoral votes and President has 213, according to CNN projections.

Pennsylvania, the state that could take Biden over the 270-vote threshold needed to win the presidency, could complete most of its outstanding counts on Thursday or Friday, officials there said.

The former vice president is only behind Trump by a little more than 18,000 votes in the Keystone State after having trailed at one point by more than half a million ballots in the hours after polls closed.

Tens of thousands of votes -- most of them from strongly Democratic areas, including around Philadelphia -- remain to be counted.

In Georgia, another state that Trump cannot afford to lose with its 16 electoral votes, Biden's mail-in ballot advantage has pulled him to within 500 votes of the President, as results came in from Fulton County around Atlanta with 99% of the state vote count reported.

Trump cannot find a route to 270 electoral votes without Georgia and Pennsylvania, so his chances of securing reelection will hinge on developments in the two states in the coming hours.

But on Thursday night, Trump effectively sent a signal that he has no intention of leaving power without a fight if he ends up losing the election.

The President's speech from the White House briefing room could end up being one of the most dangerous presidential statements in American history. In it, Trump falsely claimed that votes that were cast before and during the election, but counted after Election Day, are illegal votes.

Trump made ludicrous claims that his leads on election night shrunk because Democratic officials keep finding ballots, when in fact the counts have narrowed because election officials in many states counted the vote-by-mail ballots, which favored Democrats, after the Election Day votes, which tended to favor Republicans.

As the President spoke, the daily tally of new US coronavirus infections hit 114,876, the worst daily count ever, encapsulating how Trump's political obsessions have driven his neglect of a crisis that has killed more than 234,000 Americans.

Biden emerged in Wilmington, Delaware, for a short speech meant to project optimism, urge patience in the vote counting and to apparently create a picture of a presidency in waiting.

"In America, the vote is sacred. It is how the people of this nation express their will," he said, calling for calm and patience as the vote counting process unfolds.

The President's team, in contrast, had earlier bullishly insisted that the President would win Pennsylvania with some room to spare.

"Donald Trump is alive and well," Trump's campaign manager Bill Stepien said.

But a Trump adviser told CNN's Jim Acosta that Biden's growing momentum in the commonwealth is causing increasing worry in the campaign, conceding the margin would be "tight."

The adviser stressed Trump's future is riding on Pennsylvania.

"Pennsylvania matters," the person said, before adding, "they all matter," conceding the President can't really afford to let any of these remaining states slip away.

It has long been known that Biden would benefit from a late surge of mail-in balloting that was preferred by Democrats amid the pandemic.

The President spent months on the campaign trail, falsely blasting mail-in ballots as prone to fraud -- one reason why GOP voters have proven far less likely to use them.

In an interview with CNN's Jake Tapper Thursday afternoon, Pennsylvania Secretary of State Kathy Boockvar said "counties are furiously at work" and 'it's looking like we are ahead of schedule."

In several interviews on CNN, Democratic Sen. Bob Casey said he believed that Biden would eventually prevail in the state by up to 100,000 votes.

The story was reversed in Arizona, where several tranches of votes from Maricopa County, which includes Phoenix, narrowed Biden's lead to just under 68,000 votes, with Trump's team insisting the President will eventually prevail and keep his hopes of a path to 270 alive.

There is also a close contest in Nevada, where Biden's lead increased to nearly 12,000 votes by midday Thursday.

Democrats had the state down as a likely win but it is closer than expected. The final result will come down again to mail-in votes, which could favor Biden since thousands are outstanding in Clark County, located around Las Vegas and usually Democratic territory.

If Biden holds leads in Arizona and Nevada, he will get to 270 electoral votes and become the next President, regardless of what happens in Pennsylvania and Georgia.

CNN projects Biden will win at least three of Maine's four electoral votes, plus Wisconsin, Michigan, Hawaii, Rhode Island, Minnesota, Virginia, California, Oregon, Washington state, Illinois, New Hampshire, New Mexico, Colorado, Connecticut, New Jersey, New York, Vermont, Delaware, Washington, DC, Maryland, Massachusetts and one of Nebraska's five electoral votes.

Nebraska and Maine award two electoral votes to their statewide winners and divide their other electoral votes by congressional districts.

CNN projects Trump will win Montana, Texas, Iowa, Idaho, Ohio, Mississippi, Wyoming, Missouri, Kansas, Utah, Louisiana, Alabama, South Carolina, North Dakota, South Dakota, Arkansas, Indiana, Oklahoma, Kentucky, West Virginia, Florida and Tennessee and four of Nebraska's five electoral votes.

Trump's team, seeking to keep his slim path to victory alive, has launched a flurry of sometimes contradictory and scattershot legal challenges, without offering evidence of irregularities, demanding vote counts continue in states where he is behind and wanting them shut down in those where he leads.

"STOP THE COUNT," Trump tweeted Thursday morning, even though taking such action would freeze the race and give an advantage to his opponent and the fact that carrying out his demand would be akin to voter disenfranchisement, with tens of thousands of legally cast ballots still being counted.

As one piece of his legal strategy, the Trump campaign plans to ask the Supreme Court to intervene in a case challenging a Supreme Court decision that allowed Pennsylvania ballots to be counted after Election Day.

The justices had refused to expedite the appeal before the election and are considering whether to take up the case.

Trump and his campaign team also sought to raise doubts about how Biden made a late surge to victory in the vital state of Wisconsin, where the Democrat rose on the strength of mail-in and early votes that were counted after most of the ballots cast in person on Election Day.

The Trump campaign said Wednesday that it will demand a recount in Wisconsin while mounting legal challenges in Michigan and Georgia.

The campaign's state-by-state approach revealed the glaring inconsistencies in its strategy: it appears to be trying to stop vote counts in states where Biden is trailing, like Pennsylvania and Georgia, while demanding that all the votes are counted in states where it believes the President has a chance of catching up to Biden, like Arizona and Nevada.

Biden campaign manager Jen O'Malley Dillon has called the legal suits "meritless" and emphasized "our data shows that Joe Biden will be the next President of the United States."

Trump offered a less-than-enthusiastic endorsement of his team's legal strategy in phone calls with some of his allies on Wednesday, sounding resigned to the plan falling short and questioning why his team hadn't successfully challenged voting rules before the election, even as he remained willing to see it through, CNN reported.

A court in Pennsylvania on Thursday said Trump campaign observers could stand closer to watch ballot processing in Philadelphia -- a relatively inconsequential ruling that Trump campaign associates quickly touted as being a massive win, as votes are still being counted there that could decide the state.

No court has found wrongdoing in the ballot counting process in Philadelphia.