The Republican platform spends a lot of time on abortion, calling for extending personhood rights to "unborn children" and arguing that terminating a pregnancy threatens a woman's health and well-being.

There is one key issue that the platform is silent on: Roe v. Wade, the 1973 Supreme Court decision that legalized abortion. While Gov. Mitt Romney calls for the decision to be overturn, the Republican party does not endorse that stance.

A new working paper from  Theodore Joyce, Ruoding Tan and Yuxiu Zhang helps explain why. The paper explores what would change about abortion access if Roe were to be overturned and the issue of legal abortion returned to the states.

Women would face more obstacles to terminate a pregnancy; the average distance traveled by women obtaining abortions would increase 157 miles among 31 states expected to outlaw the procedure.

Overturning Roe, however, would by no means eliminate abortion. Even in a situation where 31 states banned legal abortion, the national abortion rate would only fall 14.9 percent.

The expectation is that, in a post-Roe world, the more liberal states, like New York and California, would continue to offer legal access -- just as they did prior to the Roe decision. Most of the south and large swaths of the Midwest, however, would likely ban the procedure.

Women in those states would likely still seek abortion services in states where they were accessible. That's what happened pre-Roe in a state like Michigan, where abortion was illegal. Women traveled out of state and, as a result, had 7.6 abortions per 1,000 women performed in New York state.

These researchers combed through the state abortion laws to understand what travel distances would look like in a post-Roe scenario. They modeled one "extreme" situation, where 31 states outlaw the procedure, as well as a more moderate version, with 17 states passing bans. Then, they modeled how far away the closest abortion provider would be for women in each of these states.

In both, the distance women in states with abortion bans increases -- an average of 157 miles in the more extreme situation and 69 miles in the moderate one. There would be huge variation in expected distance traveled, as you can see in the map below.

Texas women would face the longest travel distances, with the average woman traveling 452 miles to terminate a pregnancy. The situation would be decidedly different in Pennsylvania. It would also be expected to ban abortion but, since it's bordered by more liberal states, women would only travel an average of 42 miles.

This paper also looked at what would happen to the national abortion rate should Roe be reversed and finds that, in the extreme situation mapped above, it would drop by 14.9 percent. If only 17 states banned the procedure, traveling to obtain an abortion would become even easier, and the rate would drop 6 percent.

Even in the most extreme situation - one where 46 states ban abortion and only four allow legal access - the abortion rate only falls by 29 percent.

That helps explains why a number of antiabortion advocates don't actually favor overturning Roe: It would still allow for states to legalize abortion and, as this paper suggests, not get anywhere close to eliminating the procedure. Tellingly, the Republican platform does not call for the repeal of Roe. 

James Bopp, who drafted the Republican platform on abortion, is one prominent figure who does not like the idea of returning abortion rights to the states for this exact reason. "That puts us back to California and New York having very high abortion rates," he told me last week. "That's something I would totally oppose."