After the passage of the Taft-Hartley Act in 1947 (which was accomplished by Congress overriding President Truman's veto), Indiana enacted RTW in 1957 . . . it was repealed in 1965.  An attempt by Governor Mitch Daniels to enact it again this year was torpedoed when the Democrats in the State House fled to Illinois and Kentucky in order to deny Speaker Bosma the quorum that he needed to send HB-1468 on to the state senate for consideration.  Instead, it's been referred back to committee for further consideration.

Incidentally, the jury is still out as to whether the New Hampshire State House will override Governor Lynch's veto of HB-474, which would put RTW on the Granite State's books if the veto is successfully overridden.

Twenty-two states, plus the Territory of Guam, have RTW on their books.  Eight states (Arizona, Arkansas, Florida, Kansas, Mississippi, Nebraska, Oklahoma, and South Dakota) have gone one step further and enshrined RTW in their respective state constitutions.

The advantage of a constitutional amendment is that, unlike statutory enactment, a change to the state constitution cannot be repealed by a subsequent legislature.  The bitch of it is that a constitutional amendment will be tougher to pass, but I think that it'll be well worth the hassle.