Growing opposition to the Common Core academic standards for the nation's public schools is threatening to slow down and derail the initiative.

Forty-five states and the District of Columbia voluntarily adopted at least some of the Common Core State Standards Initiative, a state-led effort to establish a single set of educational standards for language arts and math in grades K-12. Slated to start next year, it would replace standards created by each state that Common Core advocates say are not adequately preparing students for life after high school.

The initiative, which emphasizes analytical thinking over simple memorization, is designed to raise academic standards and ensure that students are prepared for either the workforce or college when they graduate.

But now, as field testing of the new assessments starts for students in 36 states and D.C., some educators are raising concerns, saying the standards are too rigorous, especially for younger children, and parents are calling the approach "ridiculous." There have been testing problems in states that have begun implementing them, and some states are looking to change their plans. In Congress, Sen. Chuck Grassley, R-Iowa, is trying to get it defunded.

The standards promise to become a presidential campaign issue in 2016. Sens. Ted Cruz of Texas and Rand Paul of Kentucky, Tea Party favorites and possible White House candidates, portray the initiative as a one-size-fits-all nightmare and a top-down takeover of local schools, while many establishment Republicans and most Democrats support it.

"There is far more opposition and skepticism about Common Core among rank-and-file Republicans and grassroots conservatives, as well as Tea Party people, than there is support for it," said Keith Appell, a Republican strategist with CRC Public Relations. "When you discuss education on the Republican side of the aisle, people get very leery about any involvement in the national level."

Indiana Republican Gov. Mike Pence signed legislation on March 24 making his state the first to opt out of the initiative -- a move he predicted other states soon will follow.

In New York state, even some Democrats have shown concern regarding how the initiative is used, with state-level lawmakers from both parties proposing it be excluded from the state’s new teacher evaluation system.

Meanwhile, other states are trying to "rebrand" Common Core, proposing new names such as "Iowa Core" and "Next Generation Sunshine State Standards.”

But Common Core supporters, including the National Governors Association and some current and former Republican governors, say the opposition is irrational and misinformed. They say the educational benchmarks were developed as a way for states to keep the federal Education Department at arm's length. And they note participation is voluntarily.

Former Florida Gov. Jeb Bush, a staunch supporter and potential presidential contender, told the Miami Herald that critics of the effort care too much about students' self-esteem.

The initiative is backed by Republican-aligned business groups, including the powerful U.S. Chamber of Commerce and the Business Roundtable, which are part of a coalition that is running a national advertising campaign to promote it.

While the federal government officially has nothing to do with Common Core, the Obama administration is using grants to entice states to adopt the standards.

Former Georgia Gov. Sonny Perdue, a leading Common Core advocate, blamed the conservative backlash partially on the administration's support of the program.

"From a partisan point of view, the endorsement of Common Core by the federal secretary of education and President Obama have done more to damage Common Core than anything else," Perdue told Peach Pundit, a website dedicated to Georgia politics.

"I never heard any opposition or suspicion of Common Core until there was support from the president and his administration."