Federal Elections Commission (FEC) Commissioner Ellen Weintraub warned Monday that there's a "substantial chance" results for the presidential and down-ballot races may not be known on election night, as voters will likely shift to more mail-in ballots amid the coronavirus pandemic. 

"Let me just tell everybody, we're all going to need to take a deep breath and be patient this year because there's a substantial chance we are not going to know on election night what the results are," Weintraub said on CNN's "New Day."

"Probably for the presidency, but maybe for many other races that are important to people, and that's OK. If it takes a little bit longer to count all the votes accurately, that's what we need to do in order to ensure that everyone's vote counts," she added. 

Weintraub said mail-in and absentee voting will be a "preferred alternative" for many voters this year due to the coronavirus pandemic. She noted that there was an increase in absentee voting during the recent Kentucky primary compared to other years. 

President Trump has repeatedly railed against mail-in voting, making unsubstantiated claims that it leads to voter fraud.