Abortion is not, and can never be, legal, as it violates God's commands, the first law of nature, the principles of the Declaration of Independence, every clause of the stated purposes of the U.S. Constitution, and the explicit, imperative, absolute equal protection requirements of multiple amendments to that Constitution.
And yet it's still legal in the United States, regardless of your legal and Constitutional acumen.
There are two ways to fix that. One is to go for the sweeping stroke, such as the proposed human life amendment that essentially clarifies the Constitution so that it cannot be interpreted differently than you state above (an I heartily approve of). That's an idea that's been floating out there for about 3 decades now. How's it going? Hearts and minds changed yet? Is the current cop of legislators and judges more likely to see it this way than in the last 30 years?
The other path is incrementalism - using the left's own tactics against them. Every life saved is a blessing, and even the debates over these measures are helping to change attitudes. 20 weeks is absolutely an arbitrary line, and as medical science continues to improve, it's going to be more and more obvious to people that a baby at 16 weeks is little different than at 20. And then that 12 is little different from 16. Eventually, they will have to face the fact that all euphemisms aside, it's human being from the point of conception.