The question is not so easy as it seems. Let's analyze some derivatives: abortion, abortive, abortiveness, abortionist. The analysis of derivational suffixes (-ion, -ive, ive+ness etc.) helps to identify the end of the root. But what about the prefix? Is it possible to consider ab- in this word as a negative prefix, as for example in ab-normal or ab-duct?
From etymological point of view the root must be OR (from L. abortivus "pertaining to miscarriage; causing abortion," from abort-, pp. stem of aboriri "disappear, miscarry," from ab- "amiss" (see ab-) + oriri "appear, be born, arise"). What about the root in Modern English (ort or abort) ?