Bill and Hillary Clinton suggested that Mr Trump will reinforce the White House in order to prevent officials from removing him on Inauguration Day if he loses. The pair also urged the Democratic nominee Joe Biden not to concede on the election night.
The Clintons spoke in a rare interview on Instagram with American Urban Radio Washington Bureau Chief April Ryan.
Ms Clinton said: “Do not concede under any circumstance because I believe the other side is going to cheat and sneak and try everything they possibly can.”
Mr Clinton added: “He probably won’t even come to Biden’s inauguration.
“He’ll be stacking sandbags around in the White House.”
So far Mr Biden has been leading President Trump in most election polls.
During the interview, the Clintons attacked Mr Trump as Mr Clinton accused the president of manipulating voters.
He said: “It’s very important to keep in mind that beneath all these big questions you are asking are just Donald Trump’s proven ability to manipulate the psyche of the people that he is trying to scare the hell out of.
“Now, this is a person who says, ‘OK, I can’t win on my own, but people will believe whatever I tell them as long as it’s bad.”
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Voters in American will decide who their next president is on 3 November.
Mr Trump’s competitor Joe Biden is best known as Barack Obama’s vice-president.
In 2016, Ms Clinton ran for president against Mr Trump.
She also led in the polls and won nearly three million more votes than President Trump.
But Ms Clinton lost the election due to the US’s electoral college system.
It means that the number of votes is less important than where the candidate wins them.
This year Mr Biden has been leading the polls and has hovered around 50 percent in recent weeks.
Whereas Mr Trump has been hovering just above 40 percent.
But in 2016, the polls were much closer with only a few percentage points separating Mr Trump and Ms Clinton.
Both the Democrats and Republicans completed their national conventions last month.