“Let me just tell everybody, we’re all going to need to take a deep breath and be patient this year,” Ms Weintraub told CNN. “Because there’s a substantial chance we are not going to know on election night what the results are.”
Ms Weintraub said that down-ballot elections could also be hit by logistical issues as millions of US citizens opt to vote by post.
“Probably for the presidency, but maybe for many other races that are important to people, and that’s OK,” she added.
“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.”
Ms Weintraub, a New York-born Democrat, also slapped down unfounded claims by president Trump that mail ballots could fall foul to voter fraud.
“It’s been done before. It’s safe. There is no substantial risk of fraud involved in absentee voting,” she said.
Amid dire poll ratings, Mr Trump has sought to cast clouds of doubt over the legitimacy of the upcoming election, calling for it to be delayed.
But any move to delay the vote would require an act of Congress, meaning both the Senate and the House of Representatives, where the Democrats have a majority, would need to sign off on it.
Trump failed to gain support for the proposal on Capitol Hill, with senior Republicans including Marco Rubio saying the vote would go ahead.
The president’s campaign team then rowed back on the idea and sought to portray a confident mood within the camp, with White House chief of staff Mark Meadows bullishly declaring that the election would go ahead and that “the president is going to win”.
Mr Trump continues to trail Joe Biden by a considerable distance in a raft of major polls.
On Monday, Mr Biden interviewed the finalists to be his running mate and his campaign is readying to announce his choice as soon as this week, a person familiar with the process told Reuters.
Mr Biden and his potential vice president will formally accept the party’s nomination at the Democratic National Convention, scheduled for 17-20 August and he is expected to announce his pick before it begins.
The New York Times reported that Mr Biden’s campaign plans to announce a decision as soon as Tuesday, but more likely on Wednesday, citing people briefed on the selection process.
The newspaper said the committee that screened the candidates had “effectively disbanded” after completing its work. It noted that Biden has missed previous deadlines to announce his pick, and that the timeline could “slip again.”
A Biden campaign spokesperson declined a Reuters request for comment.