Clinton, at an event inside Heinz Field in Pittsburgh, painted a picture for voters where Trump would put his own financial security over what is best for voters.
"Ask yourself, so if he is sitting across the table negotiating with people from those countries, is he going to put his own financial interests ahead of America's interests?" Clinton asked. "I can tell you this, when I am sitting across the table from the Russians or anyone else, I know who I am representing: the American people."
Cuban, who endorsed Clinton earlier this year and was born and raised in Pittsburgh, went a step further, suggesting that Trump, as president, would take bribes.
"A (Vladimir) Putin, or an (Julian) Assange, would say to him, 'Donald, if you do this, I will give you $20 billion.' What is 20 billion to them? What is 30 billion?" Cuban asked. "For them, that is nothing. If Donald Trump, who rips off people for thousand -- do you think he is going to do what is right for the country, or do you think he is going to take the money?"
Cuban continued, "Do you think he has the temperament to do the right thing? Do you think he has the fortitude to do the right thing? Do you think he cares about you or his bank account?"
Cuban has been an outspoken surrogate for Clinton throughout the campaign, effectively getting under Trump's skin by questioning, among other things, whether he is actually a billionaire. Cuban attended the first presidential debate as Clinton's guest, sitting in the front row.
Cuban flew with Clinton from her event in Pittsburgh to Detroit for another rally. Speaking with reporters in-flight, the billionaire said that he was "serious" when he said Trump could be bought.
Asked by a reporter why billionaire Republican donors like the Koch Brothers wouldn't have just bribed him to get out of the race, Cuban responded, "They did."
Why didn't it work?
"Didn't offer him enough," Cuban said.
James Davis, a spokesman for the Koch brothers and their super PAC, said the charge "is completely, completely inaccurate." A message left with the Trump campaign was not immediately returned.
Seemingly feeding off of Cuban's bluntness Friday, Clinton went further than she typically goes in criticizing Trump.
"Honestly, I don't know how he lives with himself," Clinton said about Trump's tax policy. "Doesn't he see the millions of women and families struggling? Or does he just not care?"
At her rally in Detroit, Clinton said that if Trump wins next week, the United States would have a president who "has only ever been in it for himself."
"Just last year, he said again that it wouldn't have mattered if we rescued the auto industry, let it go bankrupt," Clinton said of President Barack Obama's 2009 auto bailout. "What is he talking about?"
Clinton's comments come as the campaign reaches its final sprint. Clinton will campaign Friday in Pennsylvania, Michigan and Ohio and Florida and Pennsylvania on Saturday.
Clinton aides say they are confident they will keep Pennsylvania blue Tuesday, but feel to do so, they will need to boost turnout in both Pittsburgh and the Philadelphia area. The last Republican to win the state was George H.W. Bush in 1988.
Michigan also is a state that last went to a Republican in 1988. Clinton's top aides are confident it will go to Clinton on Tuesday but said Friday's trip is an effort to spur turnout in a state where there is no early voting and everyone votes on Election Day.
"Michigan is a state we feel like we've got a lead on. We want to make sure we hold that lead, get the vote out, make sure people are enthusiastic," said John Podesta, Clinton's campaign chairman.
Clinton was joined at the Pittsburgh event by Steeler icons Franco Harris and Mel Blount. Jokingly, Clinton referred to them as her "escorts" on stage.
Polls across the country have tightened on Clinton, who once held a substantial lead nationally and in battleground states. But the candidate seemed more at ease Friday morning, joking with attendees about debating Trump three times.
"I'll tell you — some of what I heard from my opponent," Clinton said smiling, "it was really hard not to go, 'What did you say?'"