NASHVILLE, Tenn. — More than five years after U.S. governors began a bipartisan effort to set new standards in American schools, the Common Core initiative has morphed into a political tempest fueling division among Republicans.

The U.S. Chamber of Commerce leads establishment voices — such as possible presidential contender Jeb Bush — who hail the standards as a way to improve student performance and, over the long term, competitiveness of American workers.

Many archconservatives — tea party heroes Rand Paul and Ted Cruz among them — decry the system as a top-down takeover of local schools. The standards were developed and are being implemented by states, though Common Core opponents argue that President Barack Obama’s administration has encouraged adoption of the standards by various parameters it set for states applying to get lucrative federal education grants.

Tea party-aligned officials and candidates want to delay the standards or abandon them altogether in at least a dozen of the 45 states that adopted some part of the guidelines. Indiana lawmakers approved a repeal that now awaits a decision from Gov. Mike Pence.

“Common Core is like Obamacare: They passed it before they knew what was in it,” said William Evers, a Hoover Institute research fellow and lead author of a California Republican Party resolution denouncing Common Core.

To a lesser extent, Democrats must deal with some teachers — their unions hold strong influence within the party — who are upset about implementation details. But it’s the internal GOP debate that’s on display in statehouses, across 2014 campaigns and among 2016 presidential contenders.

The flap continues as students in 36 states and the District of Columbia begin this week taking field tests of new assessments based on the standards, although the real tests won’t be given for another year.

Paul, a Republican senator from Kentucky, has joined seven colleagues, including Texas’ Cruz, to sponsor a measure that would bar federal financing of any Common Core component. Florida Sen. Marco Rubio isn’t among the eight, but he had already come out against the standards. So has Rick Santorum, a 2012 presidential candidate mulling another run.

On the other end of the spectrum is Bush, the former Florida governor and Rubio’s mentor. “This is a real-world, grown-up approach to a real crisis that we have, and it’s been mired in politics,” Bush said last week in Tennessee, where he joined Republican Gov. Bill Haslam at an event to promote Common Core.

Haslam, who is running for re-election this year, is trying to beat back a repeal effort in the Tennessee legislature. “These are simply guidelines that say a fourth grader should be learning the same things” regardless of where the student lives, the governor said recently. “Historically, we haven’t been good at setting high standards.”

The National Governors Association and state education superintendents developed Common Core. Among other things, the framework recommends when students should master certain skills. For example, by the end of fifth grade, a math student should be able to graph and solve complex problems by plotting points on x and y axes. A high school sophomore should be able to analyze text or make written arguments using valid logical reasoning and sufficient evidence.

The issue presents a delicate balancing act for some governors. Bobby Jindal’s Louisiana and Scott Walker’s Wisconsin initially adopted the new standards. Now both men — possible presidential candidates — must watch as GOP lawmakers in their states push anti-Common Core bills. Jindal, who was an NGA member during Common Core’s development, told the Baton Rouge Press Club earlier this year that he’s “absolutely for rigorous standards” but “absolutely against any kind of federal takeover.” Before Wisconsin lawmakers convened, Walker announced support for rethinking Common Core. In both states, however, the anti-Common Core legislation appears stalled, as neither governor has made repeal a priority.

Establishment Republicans in Georgia, meanwhile, derailed a repeal effort in favor of a “study commission” empowered only to make recommendations. Alabama GOP leaders have held off a repeal measure, as well.

Immediate political consequences of the disputes aren’t clear. GOP officials and strategists say any fallout for them is dwarfed by Democrats’ struggle with Obama’s health care law. In the meantime, conservative candidates use Common Core as a symbolic rallying cry.

Tennessee state Rep. Joe Carr, a long-shot primary challenger to Republican Sen. Lamar Alexander, insists Common Core “is just one more overreach of a federal government that wants to insert itself into everything.” An Alabama congressional hopeful, Scott Beason, casts Common Core as liberal indoctrination. In Georgia’s crowded Republican U.S. Senate primary, Rep. Paul Broun declared in a recent debate, “I want to abolish the Department of Education and get rid of Common Core forever.” His first goal wouldn’t necessarily accomplish the second.

The arguments perplex the politicians most responsible for the plan.

Democrat Jack Markell, Delaware’s second-term governor, told the Associated Press that opponents mistakenly equate a coalition from across the nation with a federal government initiative. Markell co-chaired NGA’s Common Core panel with Republican Sonny Perdue of Georgia.

Perdue, who left office in 2011, said Common Core actually began as a pushback against federal influence because of the No Child Left Behind law, the national education act signed by President George W. Bush. Perdue said it was “embarrassing” for governors of both parties that Congress and the White House pushed higher standards before state leaders.

Perdue attributes the outcry against Common Core to Obama’s backing: “There is enough paranoia coming out of Washington, I can understand how some people would believe these rumors of a ‘federal takeover,’ try as you might to persuade people otherwise. I almost think it was detrimental … for the president to endorse it.”

Evers, the Hoover Institute fellow who was also a top Education Department appointee during the Bush administration, says it’s unfair to reduce opponents’ concerns to partisanship. He notes insufficient training for teachers expected to use new teaching methods, and he criticizes specific components. For example, some math courses are recommended for later grade levels than in standards already adopted in leading states like Massachusetts and California.

States move forward, Evers argued, because of competition. “It’s by emulation and rivalry that we have always seen advances in public education,” he said. National standards, he added, “will close the door on innovation.”