NEW YORK — Though President Obama and Mitt Romney rarely tackle the topic of abortion head-on on the stump, the void is being filled by rival advocacy groups targeting swing states with ads depicting one or the other candidate as an extremist on the divisive issue.

Obama, according to the National Right to Life Committee, is ‘‘the most proabortion president this country has ever seen.’’ Another antiabortion group, the Susan B. Anthony List, is running anti-Obama TV ads titled ‘‘Abortion Radical.’’