Chester E. Finn Jr.:

Though few Americans have ever heard of the “Common Core,” it’s causing a ruckus in education circles and turmoil in the Republican Party. Prompted by tea party activists, a couple of influential talk-radio hosts and bloggers, some disgruntled academics, several conservative think-tanks, and a couple of mysterious but deep-pocketed funders, the Republican National Committee recently adopted a resolution blasting the Common Core as “an inappropriate overreach to standardize and control the education of our children.” Several red states that previously adopted it for their schools are on the verge of backing out. Indiana has already hit the “pause” button.
What, you ask, is this all about?
Thirty years after a blue-ribbon panel declared the United States to be “a nation at risk” due to the weak performance and shoddy results of our public-education system, one of the two great reforms to have enveloped that system is the setting of explicit academic standards in core subjects, standards that make clear what math youngsters should know by the end of fifth grade, what reading and writing skills they must acquire by tenth grade, and so on. (The other great reform: school choice.)
Up to now, individual states set their own academic standards. A few did this well but most, according to reviews undertaken by the Thomas B. Fordham Institute and others, faltered badly, putting forth vague expectations that lack content and rigor, are unhelpful to teachers and curriculum directors, and often promote left-wing dogma. Even the good ones differ so much from state to state that school and student performance cannot be compared around the country, much less with other lands.