The American political tradition is for presidential rivals to bury the hatchet once the primary season is over, and throw their joint weight into the battle against the opposite camp as election day draws near.
But wiping the slate clean might not come easy. Here are some of the harsh words they've fired at one another.
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"I'm so sick of the Sanders campaign lying about me. I'm sick of it," Hillary Clinton told a climate activist in New York in March, after she accused her of accepting donations from fossil fuel lobbyists. The candidate rarely let her anger slip so unabashedly, but animosity between the rivals has risen throughout the Democratic primaries.
"His campaign, which he claims to be a positive campaign, has engaged in artful smear, innuendo and insinuation and enough is enough," she said in a television interview in February.
But she's aimed her toughest words at the heart of her socialist opponent's message: She says it's not up to snuff. "I think he hasn't done his homework and he'd been talking for more than a year about doing things that he obviously hadn't really studied or understood, and that does raise a lot of questions," she said in the lead-up to the April New York primary.
Addressing Sanders' main social and economic reform plans, she says: "The numbers just don't add up."
Bernie Sanders has crusaded for more than a year to convince Democrats that Hillary Clinton, despite her experience, would make a poor leader. His first piece of evidence is her 2002 Senate vote in favor of the Iraq War -- a vote she later said she regretted.
"I do question her judgment. I question a judgment which voted for the war in Iraq -- the worst foreign policy blunder in the history of this country," said Sanders in the last Democratic debate held in New York in April.
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"Maybe Secretary Clinton might want to apologize to the families who lost their loved ones in Iraq?" he said in an interview.
Clinton's historical connection to Wall Street is Sanders' other main talking point. "I don't believe that she is qualified if she is through her Super PAC taking tens of millions of dollars in special interest funds," he said in April.
And when Clinton claimed she was certain of being the party nominee well before the primaries were over, Sanders brushed it off: "Just a tinge of arrogance there, I think."
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