We support constitutional state sovereignty including nullification of federal oversteps.
We disagree with Roe vs. Wade and Doe vs. Bolton as “settled law.” Under the Tenth amendment, these Supreme Court decisions have no authority over the states.
Even as anti-federalist and strong supporters of state's rights, James Madison and Thomas Jefferson balked at the idea of nullification, which is the idea that a state can ignore a federal law. Jefferson was more open to the idea than Madison, having wrote favorably of the concept of nullification on at least one instance. But Madison was successful in talking Jefferson down from the idea. And while any flirtation with nullification was probably partly rooted in their personal philosophies, I think a lot of it was also based on their love for Virginia and their distrust of NY and other northern states. (Footnote: I got this from the book "Madison and Jefferson". If I can remember I'll try to cite a page # for reference.)
That's not to mention that they are just plain wrong about the scope of the 10th amendment. Technically no law or SC decision is settled. There is always the possibility that they can be changed. But federal laws and SC absolutely have authority over the states. Apparently the Iowa GOP doesn't know that we fought a civil war to decide that issue. So not only are they technically wrong, they don't really have much of an argument if they go to the founders for validation of their claims. Even if they did and people like Madison or Jefferson agreed with them, I would just go to my "But, Hamilton" argument.
Aside from the merits of nullification, which I find ridiculous, I don't think they have fully thought through the implications. Granted, this is the Iowa GOP. So they aren't really concerned with what other states are doing. But let's say they were right and they could nullify federal laws and SC decisions. Well, that would also mean that liberal leaning states could do the same. So if Republicans were somehow able to outlaw abortion at the federal level, liberal states would be able to nullify that and allow the people of their state to perform abortions. And I highly doubt they would be ok with that. What they really want is just the power to ignore laws they don't like while not allow other people to do the same.
Update with a quick thought: If you think state have the right to nullify federal law, why not take the stance that individuals have the right to nullify state and federal law?