Donald TrumpDonald John TrumpBusiness, ballots and battling opioids: Why the Universal Postal Union benefits the US Sanders supporters cry foul over Working Families endorsement of Warren California poll: Biden, Sanders lead Democratic field; Harris takes fifth MORE said Monday that his rivals for the GOP’s 2016 presidential nomination are poorly defending Americans’ gun rights.

Trump, who has seen Ben CarsonBenjamin (Ben) Solomon CarsonTrump heads to heart of resistance in California Trump awards Yankees legend Mariano Rivera the Medal of Freedom HUD watchdog finds no misconduct by Carson in furniture controversy: report MORE rise ahead of him in two recent polls in Iowa, lashed out at his conservative rival as well as frequent foil Jeb Bush.

“My opponents are weak on the Second Amendment,” he told host Matt Lauer during a town hall event televised on NBC’s “The Today Show.”

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“Carson is weak on the Second Amendment. Bush is weak on the Second Amendment,” he said.

Trump also argued that stricter gun control is not a necessary policy measure given the existing firearms rules.

“We have tremendous regulations already,” he said. “There are plenty right now. Government does a terrible job enforcing them.”

Iowa's first-in-the-nation caucuses are less than 100 days away, and Carson may now be the favorite to win them. Iowa GOP voters often gravitate to social conservatives, and Trump has seen Carson overcome him in polls by Quinnipiac and Bloomberg that were released last week.

Bush, on the other hand, has sought to cut costs as questions arise about whether his flagging campaign can make a comeback. Trump repeatedly attacked Bush on Monday, perhaps believing he can be pushed from the race.

Trump’s remarks on guns follow a mass shooting at Umpqua Community College in Roseburg, Ore., earlier this month. Christopher Harper Mercer, 26, reportedly killed nine students and wounded nine more before turning a weapon on himself.

Trump charged that the rash of mass shootings across the country stem more often from untreated mental illness than lax gun control laws.

“This is a mental health problem,” he said during Monday's event in Atkinson, N.H. “We have a huge mental health problem.”

“These people are crazy,” Trump added of potential killers. “They will find a way to get guns or something else.”