Biden was declared the winner of Wisconsin's 10 electoral votes and Michigan's 16 electoral votes by outlets including NBC News. The Trump campaign was planning to request a recount in Wisconsin, as well as file suit to stop counting in Michigan.
"There have been reports of irregularities in several Wisconsin counties, which raise serious doubts about the validity of the results," Trump campaign manager Bill Stepien said, though he declined to offer evidence. "The president is well within the threshold to request a recount and we will immediately do so."
The state put Biden's electoral vote count at 253 to Trump's 213.
Though his lead in the electoral college increased Wednesday, Biden said he's not prepared to declare victory and said all votes should be counted in remaining states.
Six states were undecided as of midday Wednesday -- Arizona, Nevada, Pennsylvania, Georgia, North Carolina and Alaska, as well as a single electoral vote yet to be determined in Maine.
Wisconsin law doesn't provide for an automatic recount, but candidates may request one if the candidates are within 1% of the vote of each other. Some 3.24 million Wisconsin voters cast ballots Tuesday.
Meagan Wolfe, Wisconsin's chief elections official, said all votes -- except for those by a town of 300 people -- have been counted in the state, but they've yet to be finalized.
"Now we're in the important process of triple-checking the results," she said. Many votes were left to count in the key battleground states of Georgia, North Carolina, Pennsylvania and Michigan. Some of those are expected to come from Democratic-leaning cities and counties such as Pittsburgh, Detroit and Atlanta.
In Pennsylvania, with about 80% of votes counted, Trump leads by about 450,000 votes.
In Georgia, with 99% of votes counted, Trump leads by about 85,000. The president's lead in North Carolina is about 76,000, with 100% counted.
Biden holds leads in Arizona, Nevada and Michigan, which carry a combined 33 electoral votes.
The slow process of counting ballots, especially early vote and mail ballots, was expected to shift the margins in Biden's favor. Pennsylvania may not report large chunks of its results for days because of an abnormally large number of mail-in ballots driven by the COVID-19 pandemic.
Stepien told reporters Wednesday the president's team remained "confident on our pathway."
"We are confident in our math. We are viewing some of these races as math equations," he said.
Stepien cited tight races in Arizona and Pennsylvania, which he predicted Trump would win.
"Late-arriving votes cast closest to Election Day are the ones being counted now" in Arizona, he said, predicting that a majority of them would be for Trump. Biden campaign manager Jen O'Malley Dillion was also confident Wednesday.
"I want to share with all of you that Joe Biden is on track to win this election and he will be the next president of the United States," she said at a briefing, predicting he would win his native state of Pennsylvania.
"We see 1.4 million outstanding ballots that will be counted over the coming days, most of them projected to be from Democratic-heavy areas and mail-in vote," she added.
Pennsylvania Secretary of State Kathy Boockvar vowed that a true final count is coming.
"We are going to accurately count every single ballot," she told reporters in Harrisburg.
Pennsylvania Gov. Tom Wolf said about 3 million mail ballots were received and they are being counted. He warned that a final result might not be available Wednesday.
"The most important thing is that we have accurate results ... even if that takes a little longer than we're used to," he said.
In Nevada, elections officials said no more results will come until 9 a.m. Thursday.
The state elections division said they still have to count mail ballots received on Election Day, mail ballots postmarked by Nov. 3 and received over the next week and provisional ballots.
In Michigan, Secretary of State Jocelyn Benson said "tens of thousands" of ballots remained to be counted, including from some the state's largest cities, such as Detroit and Grand Rapids.
"I'm optimistic that by the end of the day, the majority of our ballots will be tabulated and we'll be much closer to having a full, if not a full and complete, unofficial result to announce," she told reporters Wednesday.
Likewise in Georgia, Secretary of State Brad Raffensperger said the state will be "pushing really hard" to count "all legal votes" by the end of Wednesday.
More than 200,000 mail-in and absentee ballots remained to be counted, he told reporters. Even if all the ballots are not actually counted by the end of the day, he said he hoped a winner could be announced before the day is over.
Overnight, Trump attempted to claim victory.
"We were getting ready for a big celebration. We were winning everything, and all of a sudden it was just called off," he said early Wednesday, adding that he wants courts to step in and stop the counting.
Biden encouraged patience and said he's in a good position to win.
"It's not my place or Donald Trump's place to declare the winner of this election. It's the voters' place," he tweeted.
"We knew this was going to go long, but who knew we were going to go into maybe tomorrow morning or even longer. But look, we feel good about where we are. We really do."
"We won't rest until everyone's vote is counted," he added in another tweet.
Biden will give an address later Wednesday, his campaign manager said.
The race has been called for Trump in Kentucky (8 electoral votes), Mississippi (6), Missouri (10), South Carolina (9), Louisiana (8), Alabama (9), Kansas (6), Utah (6), South Dakota (3), North Dakota (3), Indiana (11), Tennessee (11), West Virginia (5), Iowa (6), Arkansas (6), Wyoming (3), Oklahoma (7), Idaho (4), Ohio (18), Florida (29), Texas (38) and Montana (3).
Four of Nebraska's five electoral votes have also been called for Trump and Biden is leading to take the other.
Biden is the projected winner in Michigan (16), Wisconsin (10), California (55), New York (29), New Jersey (14), Illinois (20), Oregon (7), Washington (12), New Mexico (5), Colorado (9), Connecticut (7), Washington, D.C. (3), Massachusetts (11), Maryland (10), Delaware (3), New Hampshire (4), Virginia (13), Minnesota (10), Rhode Island (4), Hawaii (4) Vermont (3) and in three of Maine's four congressional districts.