With 100% of the precincts reporting, Underwood is leading by 584 votes.
Oberweis declared victory in a statement and on social media, but there are still mail-in ballots that have not yet been counted that Underwood said she wants to see in the final tally.
ABC 7 Chicago news partner the Daily Herald reported Underwood's gain come from new ballot counts filed Saturday night by the Lake County Clerk's Office in Illinois.
In a statement earlier this week, Underwood said, "Jim Oberweis doesn't get to call this election: the voters do. There are thousands of votes that have yet to be counted. We appreciate every voter who made their voice heard, and our county clerks and election officials must count every ballot in as expeditious and transparent a manner as is possible. Based on publicly available data, we remain confident that once ballots are counted, this race will reflect that the voters have reelected Congresswoman Lauren Underwood."
Underwood said she planned to wait until all votes were counted, based on trends that show that the first day-of in-person votes tend to skew more Republican, while early and mail-in votes, which are counted second, skew more Democrat. Considering the record-setting number of mail-in and early votes in Illinois this year, Underwood said she would wait to see what the final tally is.
RELATED: Election results: Counting ballots continues, even in Illinois
She said she is overall encouraged that there was such a high voter turnout.
"You can't always know an hour or two in. And that's OK. Because just like voters wait hours standing in line to vote, and they should not have to but they do because it's that important, we will wait for every ballot to be counted," Underwood said.
Oberweis was equally optimistic.
"I'm afraid to get too excited, but it looks like it's gonna be a really good night," he said Tuesday.
Voters are still waiting to find out who will represent them.
When Underwood won the 14th congressional district in 2018 she made history, becoming the first woman and the first African American to represent the district. A registered nurse, the political newcomer was part of a wave of dozens of Republican districts across the country that flipped to Democrat.
Oberweis is hoping he wins and returns the district to a Republican stronghold.
Underwood raised more than $7 million during her campaign, while Oberweis raised about $2.5 million.
The 14th District covers portions of seven different counties and typically leans Republican.