MIAMI — Why is Donald Trump telling supporters the election is rigged?

The answer, according to Hillary Clinton's running mate, is as simple as a word: He's realized that he is, as Tim Kaine put it here in plain terms on Saturday night, "a loser."

"Donald Trump realizes he's losing, and Hillary Clinton definitely made him realize in that first debate that he was a loser," Kaine told a crowd of 150 at Miami Dade College.

“He’s blaming everybody. It’s the media’s fault, it’s the GOP’s fault — it’s everybody’s fault,” Kaine went on. “Now that he thinks he's gonna lose, he’s going around and saying, 'Oh, the whole thing’s rigged. It’s just rigged against me. Poor me!’"

The Virginia senator offered the Clinton campaign’s first extended response to Trump’s increasing claims that the election is “rigged,” and that a corrupt system is attempting to sink his campaign with the sexual misconduct allegations in recent days.

It was Kaine’s fifth trip to Florida, the crucial battleground state, where he addressed the small outdoor crowd in black Ray-Bans from the bed of a pickup truck, framed by a hand-painted Clinton-Kaine-themed graffiti backdrop. (The mural, bearing flowers and the phrases “Love Trumps Hate” and “Vote Florida,” was completed overnight by artist Felipe Lagos, along with the help of a few Clinton aides.)