One can not be Democrat unless one supports abortion on demand, without restrictions. It would be like wanting to join the Nazi party (in Germany, during the Third Reich) but opposing persecution of Jews, or like join the Communist Party of the Soviet Union but favoring private property.
In all fairness, other cultures have allowed murder of children, such as the toleration of infanticide for a variety of reasons (e.g. birth defects), and in Rome a head of the family could order the execution of any descendant at any age. The American left feels that mothers should be able to kill their babies at least until birth, and some argue longer. If this boths you, become a Republican.