Halfway through Thursday night’s GOP presidential debate, Fox News moderator Bret Baier asked Jeb Bush to defend his support of the Common Core State Standards, national academic standards that have become deeply unpopular among some conservatives — and many of the candidates onstage.

The audience at Quicken Loans Arena in Cleveland booed when it heard Baier say “Common Core” as part of the question.

Bush — who has long supported the Common Core and has offered nuanced defenses of it during the early stages of the presidential campaign — said that he is not in favor of federal control over education, but then quickly pivoted away from Common Core to his record as a proponent of school choice — a favored conservative policy — as two-term governor of Florida from 1999 to 2007.

“I’m for higher standards measured in an intellectually honest way, with abundant school choice, ending social promotion,” Bush said. “And I know how to do this because as governor of the state of Florida I created the first statewide voucher program in the country, the second statewide voucher program in the country and the third statewide voucher program in the country.”

Bush then took a swipe at teachers unions, also a far-from-controversial move for a candidate seeking votes from the Republican base:

“And we had rising student achievement across the board, because high standards, robust accountability, ending social promotion in third grade, real school choice across the board, challenging the teachers union and beating them is the way to go.”

Baier then turned to one of the other Floridians on stage, Sen. Marco Rubio, and asked him to explain why Bush is wrong on Common Core.

Rubio said he believes in “curriculum reform” at the state and local levels. But he said the big problem with Common Core is that the federal government is trying to “force it down the throats of our people in our states,” he said, to applause from the Cleveland crowd. (Though Ohio is still teaching Common Core-based concepts, it dropped out of one of the testing consortia tied to the Common Core last month, joining numerous other states.)

“The Department of Education, like every federal agency, will never be satisfied,” he said. “They will not stop with it being a suggestion. They will turn it into a mandate.”

The Common Core aims to create some amount of uniformity in terms of what children learn in the nation’s public schools, with an eye toward college and career readiness upon high school graduation. Though some have associated the standards with federal overreach — the Obama administration supports the use of the Common Core — it is not a federal program.

A bipartisan group of governors and state education chiefs created the Common Core in 2010 as a way to bring consistency into K-12 academic standards, which have varied wildly from state to state. The standards spell out the skills and knowledge students should possess by the end of each grade. They are not curriculum — states and schools decide how to teach to the standards and the kinds of materials to use.

Bush, responding to Rubio, walked a delicate line that he has tried to maintain since Common Core became poisonous in conservative circles.

“If states want to opt out of Common Core, fine. Just make sure your standards are high,” Bush said. “If we are going to compete in this world we’re in today, there is no possible way we can do it with lowering expectations and dumbing down everything.”