After an extraordinary night that put the country’s democracy to the test, the presidential election on Wednesday rested on the results in several crucial battlegrounds that were favoring Joseph R. Biden Jr., who picked up Wisconsin and was holding slim leads in Michigan and Arizona, all states President Trump carried in 2016.
With votes still being counted from coast to coast, Mr. Biden was fending off Mr. Trump in the West, where late vote counts shrunk the former vice-president’s margin, and he was steadily building an advantage in the Great Lakes states that tipped the presidency four years ago.
In midafternoon, The Associated Press declared Mr. Biden the victor in Wisconsin, where he held a lead of less than 1 percent. He also held a lead of 93,000 votes in Arizona, but Republicans there argued that the remaining uncounted ballots could erase the former vice-president’s advantage. Mr. Biden was leading by just 8,000 votes in Nevada, where no news organization has made a declaration, but a number of uncounted mail ballots were expected to favor him there.
If Mr. Biden holds off Mr. Trump in Arizona and Nevada, and maintains his lead in Michigan, he would have enough electoral votes to claim victory.
But as one of the most unusual presidential elections in history stretched into its second, but certainly not last, day of counting, Mr. Trump himself appeared determined to stoke an atmosphere of anxiety and political friction: After using an election-night speech to attack the integrity of the vote, the president kept up his barrage on Wednesday, amplifying baseless conspiracy theories about the accumulating votes for Mr. Biden in slow-counting states.
But while Mr. Trump has called for an end to ballot counting — effectively demanding that millions of voters be disenfranchised — — and threatened to go to court over the matter, his campaign did not launch any legal actions on Wednesday, and it was not clear that the Trump team had an actual theory of how to turn the president’s grievances into litigation.
Around 2 a.m. on Wednesday Mr. Trump appeared at the White House to falsely insist he had won and demand the votes stop being counted. The president railed on Twitter Wednesday morning about the votes Mr. Biden gained overnight.
“How come every time they count Mail-In ballot dumps they are so devastating in their percentage and power of destruction?” he wrote, as though giving voice to his inner monologue as he watched his opponent take the lead in two pivotal Midwestern states.
Having picked up Wisconsin, the Biden campaign turned its sights to Michigan, where he was leading by 37,000 votes with 97 percent of the ballots counted. Republican officials there and in Wisconsin had refused to accommodate the shift toward voting by mail and allow such ballots to be counted before Tuesday, so Mr. Trump had established an early lead before the absentee votes were tallied.
But after a police-escorted Milwaukee County official revealed the full mail ballot returns from there in a suspenseful, middle-of-the-night announcement, Mr. Biden took the lead. And after the absentee votes were counted in some of Michigan’s largest counties, Mr. Biden claimed an even larger advantage there by Wednesday morning.
The Wisconsin margin is small enough to go to a recount, but, as former Gov. Scott Walker of Wisconsin, a Republican, put it: “If it holds, 20,000 is a high hurdle.”
Mr. Trump’s campaign manager, Bill Stepien, released a statement on Wednesday afternoon promising to request a recount in the state.
Even though Wisconsin and Michigan loomed as the most critical states, North Carolina, Georgia and Pennsylvania were also still up in the air on Wednesday.
North Carolina was tilting toward Mr. Trump, who was clinging to a 77,000 vote lead, with 95 percent of the votes counted. But there was more uncertainty in Georgia, where a few hundred thousand outstanding ballots, mostly in the Atlanta area, offered Mr. Biden a chance to narrow Mr. Trump’s 87,000 vote lead.
Pennsylvania was perhaps the biggest question mark, in part because it had only counted about 80 percent of its votes by midday Wednesday and was expecting more mail in ballots to arrive this week. Both campaigns used conference calls with reporters to express confidence in their potential for victory, even though Mr. Biden had the more obvious path.
Jennifer O’Malley Dillon, his campaign manager, said they expected Michigan and Wisconsin to be called for them today and projected confidence they’d claim Pennsylvania later in the week.
“We think that this is already a foregone conclusion, but there are still a couple of states that are still in play, that are a little bit closer, but we think it is possible could push us even further beyond 270,” said Ms. O’Malley Dillon, after noting their leads in Arizona, Nevada, Wisconsin and Michigan.
In his remarks, Mr. Stepien pointed out that Wisconsin would go to recount and projected confidence in retaining Georgia. But Mr. Stepien insisted Nevada’s mail-in ballots would break to Mr. Trump, even though they’ve overwhelmingly favored Mr. Biden, and said the president would win Pennsylvania by about 40,000 votes.
“If we count all legal ballots, the president wins,” he said.
On Tuesday night, Mr. Trump dashed Democrats’ hopes of picking up both Florida and Ohio, two swing states that have tilted to the right in recent years, and that Mr. Trump carried four years ago. He also turned back a challenge from Mr. Biden in Iowa, a smaller state where Mr. Biden made a late effort to pick up its six Electoral College votes.
In Georgia, there appeared to be a large number of uncounted ballots in the Atlanta metro area, and those votes were expected to tilt solidly to Mr. Biden. And in a number of the state’s rural counties, Mr. Biden was slightly outperforming the margins posted by Stacey Abrams, a Democrat who lost a race for governor there two years ago by about 55,000 votes.
Vote-counting was moving relatively slowly in some battleground states on Tuesday night because of the scale of the turnout, a backlog of absentee ballots received by mail and scattered problems with processing the vote. And each state handled the counting and releasing of its ballots differently.
Ohio, for example, released the results of all of its mail ballots after the polls closed — making the state seem to tilt toward Mr. Biden until more Election Day votes were cast. Similarly, Michigan released its day-of votes in the first hours after polls closed, making it seem that Mr. Trump enjoyed a wide advantage in a hotly contested state.
Much of the uncertainty hanging over the election arose from the inconsistent or patchwork array of state-level policies hurriedly put in place to enable voting amid a public health disaster. In a number of states, like Pennsylvania and Michigan, local Republican officials blocked Democrats’ efforts to make it easier to count ballots cast before Election Day, raising the possibility of a drawn-out count in some of the most important battlegrounds — the very occurrence Mr. Trump protested Wednesday morning.
Democrats feared that in some cases a Supreme Court now dominated by conservative justices could ultimately limit vote-counting in a way that would aid Mr. Trump, a possibility the president raised in his remarks.