Jonathan Adler is mostly right but ultimately wrong in his argument that the federal government lacks the power to regulate abortion, and that such power is reserved for the states. However, he seems to slide past the most powerful and obvious counter-argument: the Supreme Court has injected itself into the issue (and it's part of the federal government).
(Obviously I'm not a lawyer, but I believe that abortion is a moral and political question, not primarily a legal one. I'm not trying to create an air-tight legal position that supports any specific abortion restrictions.)
Relevant legal questions -- how to define murder, when to excuse the taking of life as defensible or otherwise permissible, even defining what constitutes the end-of-life for medical and other purposes -- have always been matters of state law. Drawing such lines necessarily involves drawing distinctions that will please some and offend others, but that hardly creates an equal protection problem, let alone justify federal legislation. Again, where protected classes are not involved, a state's decision to draw different distinctions than would the federal government, even on matters involving life and death, is insufficient to justify a federal law.
Note there the invocation of "protected classes" -- who decides what the protected classes are? Can't Congress? Of course it can; Congress (with sign-off from the President) can decide that unborn babies are a protected class. Most of the protected classes were created by the Civil Rights Act of 1964, which was was passed by Congress and signed by President Johnson.
Glenn Reynolds also argues that Republicans should oppose abortion legislation because they support limited government. I respect the Instapundit a lot, but I think he misses the same element that Mr. Adler did.
One such conflict is likely to appear this week, when the House is expected to vote on a 20-week limit on abortions. Such a limit polls well-- Americans are much more supportive of early abortionsthan late-term abortions -- and would still leave the United States with more-liberal abortion laws than nearly all of Europe. Even so, the Republicans need to be asking themselves -- and the Democrats need to be asking them, too -- where, exactly, Congress gets the power to limit abortions to 20 weeks?
Where did the Supreme Court get the power to enable abortions? Article 3, Section 2 of the Constitution says that Congress may limit the jurisdiction of the Supreme Court (except for "Cases affecting Ambassadors, other public Ministers and Consuls, and those in which a State shall be Party"). So clearly Congress could remove abortion from the jurisdiction of the Supreme Court if it wanted to. That's de facto power to regulate abortion without interference from the courts, if Congress chose to exercise it.
As I wrote at the top, I'm not skilled enough to make an air-tight legal argument. But I know that if the law allows the slaughter of millions of babies every year then the law is wrong.