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The spot runs down clips of Cruz referring to Social Security as a "Ponzi scheme," calling to prosecute Planned Parenthood, promising to eliminate five government agencies, defending his role in the 2013 government shutdown and touting gun rights.

"Donald Trump may get all the attention but Ted Cruz is the GOP frontrunner in Iowa and he’s so extreme that he might even be worse than Trump,” Justin Barasky, spokesman at the Priorities USA super-PAC, said in a statement.

“Ted Cruz is so extreme, dangerous and wrong that he makes Donald Trump look like Donald Duck.”

The online ad is part of the super-PAC's larger $1.5 million ad buy used to issue blanket attacks on Republican presidential candidates ahead of the general election.

Previous attacks questioned why the GOP field continues to say they'll support the GOP nominee for president, even if it is Trump, but this is the first to exclusively target a Republican rival.