"I don't want to scare people, but I want you to be prepared. I have every reason to believe that Trump is not going to go, you know, silently into the night if he loses," she said at the 19th Represents Summit. "He's going to try to confuse us, he's going to try to bring all kinds of lawsuits, he'd got his crony Attorney General William Barr ready to do whatever is necessary."
Democrats have voiced concerns that Mr Trump is preparing to contest the election by casting doubt on its legitimacy.
The president has already attempted to derail vote-by-mail by drawing a meaningless distinction between it and absentee voting, and - more directly - by blocking a coronavirus stimulus package that would have expanded voting and funded the US Postal Service.
He has been claiming for months that mail-in voting will lead to rampant voter fraud - an allegation that is unsupported by evidence and rejected by election officials - and said a vote-by-mail election would be the greatest election tragedy "in history."
In addition to attacking mail-in voting's credibility and funding, Mr Trump has suggested he might not leave office if he feels the results of the election are questionable.
"I have to see. Look, you - I have to see," Mr Trump said. "No, I'm not going to just say yes. I'm not going to say no."
He also suggested changing the date of the election - something he does not have the power to do - until the coronavirus pandemic has subsided or a viable treatment or vaccine is available.
Ms Clinton said she wouldn't be opposed to working in presumptive Democratic presidential candidate Joe Biden's cabinet should he win in November. However, she said she wasn't giving it much thought and was focusing on the election.
"I can't even think yet about the administration. I'm ready to help in any way I can, because I think this will be a moment where every American - I don't care what party you are, I don't care what age, race, gender, I don't care - every American should want to fix our country," she said. "So if you're asked to serve, you should certainly consider that."
Mr Biden has also expressed concerns that Mr Trump will try to interfere with the legitimacy of the election.
In April, he warned that Mr Trump might try to move the election, and more recently, expressed fears that the president would "try to steal this election."
During an interview on The Daily Show with Trevor Noah, Mr Biden said he was "absolutely convinced" that the US military would remove Mr Trump from the White House if he lost but refused to relinquish his position.