You are completely misconstruing the words of the Constitution. The operative words of the “Elastic Clause” [1.8.18] are “necessary and proper for carrying into execution the foregoing powers.” In other words, any legislation enacted under the Elastic Clause must be grounded in one of the previously enumerated powers.

Your claim is that a law legalizing abortion would be authorized under [1.8.1], which you quote only partially and out of context. The full clause reads,

“The Congress shall have power to lay and collect taxes, duties, imposts and excises to pay the debts and provide for the common defense and general welfare of the United States.”

The clause is about taxation; it empowers the Congress to impose taxes for the common defense and general welfare, not just do whatever it wants for those purposes. Legalizing abortion has nothing to do with taxes, so it is not within the purview of [1.8.1].

Besides that, it’s hard to see how such a law could be devised. Congress can criminally prohibit something (tax evasion, witness tampering, whatever) by law, but there is no way to pass a law permitting something. The goal you’re trying to achieve is to prevent the states from outlawing abortion, but there’s no way to do that legislatively, short of a constitutional amendment. Your whole argument makes no sense.

Don’t get me wrong, I’m not saying I want to outlaw abortion. I want it to be a legally available option. I just don’t think it’s something Congress has the power to enact.