Even before Michigan passed a right-to-work law in 2012, U.S. car manufacturing was drifting south to places like Tennessee that had long had the law. A study shows just how far the shift has happened: An estimated 70 percent of domestic car and car parts manufacturing is now located in right-to-work states.

That is the finding of a National Institute for Labor Relations Research analysis released Monday. Right-to-work laws prevent unions and businesses from negotiating contracts that require all of the business' employees to either belong to the union or at least pay it a fee. Currently 24 states have versions of the law.

NILRR concedes that a large portion of this shift comes from the fact that Michigan and Indiana both became right-to-work states in 2012. Those states remain, respectively, the number one and number two states for auto manufacturing.

But even excluding them from the total and considering just the 22 states that had adopted right to work prior to 2012, their share of auto manufacturing had grown from 36 percent in 2002 to 52 percent a decade later.

In those same 22 states, real manufacturing GDP grew by 87 percent over the same 2002 to 2012 period. Meanwhile, it fell by a modest 2 percent in states that allowed "closed shop" union rules, excluding Michigan and Indiana.

"The overwhelming advantage Right to Work states have enjoyed over forced-unionism states in attracting automotive manufacturing investment ought to put the burden of proof on Big Labor legislators in forced-unionism states like Kentucky, Missouri and Ohio who claim it makes no difference to companies considering new plant construction or expansions whether unionism is voluntary or not," NILRR noted.