Story highlights Carson was the only candidate to beat Trump in a head-to-head, the poll found.

It's a new high for Trump, narrowly topping the 28% support he registered in a Quinnipiac poll late last month.

Washington (CNN) Donald Trump is expanding his strong lead over the other GOP presidential candidates nationally, according to a new poll, but Ben Carson is also posting a big jump in support.

In the first national Monmouth University poll out since the first Republican presidential debate, released Thursday, the real estate mogul is showing a commanding lead at 30% support. That's up 4 points from a poll taken just before the debate last month, but within the poll's margin of error.

It's also a new high for Trump, narrowly topping the 28% support he registered in a Quinnipiac poll late last month.

But Carson, a neurosurgeon, posted the biggest gain in support since August, jumping 13 points to second place. He is holding down 18% support now among Republicans. That continues a trend of recent polls that show him moving up the list.

It's also Carson's highest recorded number in a national poll, topping a 13% mark in a Fox news poll this spring.

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