The Republican Presidential front-runner is taking direct aim at a top rival... Businessman Donald Trump questioning why Ben Carson is so popular.

FOX News' Jared Halpern has more from Washington:

At a rally in Fort Dodge, Iowa, Donald Trump cast a wide net of attacks, but none as stinging as those leveled at Dr. Ben Carson... Likening the retired neurosurgeon's self-described pathological temper as a teenager to that of a child molester...

(Trump) "If you're a child molester, a sick puppy, you're a child molester, there's no cure for that."

Today a response from Carson...

(Carson) "People will have an opportunity to look at all of us to assess what kind of people we are."

Nearly every recent poll shows Trump and Carson neck-and-neck at the top of the Republican field.

In Washington, Jared Halpern, FOX News.