Can't democrats just legislate abortion through congress.
They can pass legislation, but the Supreme Court has ability to strike down a Congressional act (or even state law) as unconstitutional via judicial review (though it's relatively rare for obvious reasons -- the whole equal branches thingy *cough*).
Per uscourts.gov:
The best-known power of the Supreme Court is judicial review, or the ability of the Court to declare a Legislative or Executive act in violation of the Constitution, is not found within the text of the Constitution itself. The Court established this doctrine in the case of Marbury v. Madison (1803).
In this case, the Court had to decide whether an Act of Congress or the Constitution was the supreme law of the land. The Judiciary Act of 1789 gave the Supreme Court original jurisdiction to issue writs of mandamus (legal orders compelling government officials to act in accordance with the law). A suit was brought under this Act, but the Supreme Court noted that the Constitution did not permit the Court to have original jurisdiction in this matter. Since Article VI of the Constitution establishes the Constitution as the Supreme Law of the Land, the Court held that an Act of Congress that is contrary to the Constitution could not stand. In subsequent cases, the Court also established its authority to strike down state laws found to be in violation of the Constitution.
Before the passage of the Fourteenth Amendment (1869), the provisions of the Bill of Rights were only applicable to the federal government. After the Amendment's passage, the Supreme Court began ruling that most of its provisions were applicable to the states as well. Therefore, the Court has the final say over when a right is protected by the Constitution or when a Constitutional right is violated.
Source: https://www.uscourts.gov/about-federal-courts/educational-resources/about-educational-outreach/activity-resources/about
(Legal and Constitutional buffs, please feel free to correct me if my expert-memory-recall and expert-Googling is wrong). 
