Litigants have filed another challenge to the Common Core, this time in Missouri.  The theory there is particularly unique.  They charge that state funding of the consortium that is developing Common Core standards and assessments amounts to an "illegal interstate compact" and cedes state sovereignty over education to the consortium.  They also charge that the U.S. Department of Education has illegally funded the consortium: $360 million to Smarter Balanced and the Partnership for Assessment of Readiness for College and Careers (PARCC), which are developing the standards.  The lawsuit alleges this funding was not authorized by Congress.  

I have not investigated this latter claim, but am skeptical, given that the funds flowed through the American Recovery and Reinvestment Act, which gave the U.S. Department of Education significant discretion in awarding grants to promote education innovation.  That level of funding to Common Core developers, however, would give added support to the argument that the college and career readiness requirements in Race to the Top and No Child Left Behind Waivers were de facto requirements that states adopt the Common Core.  In other words, the Department funded a private group to develop standards and then required states to adopt standards that could be found in only one place: the place that the Department funded.

More on this argument here.