Their argument is that the Tenth Amendment prohibits the federal government from doing anything except for the very few things specifically mentioned in the body of the Constitution. The Tenth Amendment reads, in it's entirety:
The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.
That's it. "Powers" is a very vague word. They choose to define it as meaning only those very specific tasks and responsibilities mentioned in the body. If the Constitution doesn't specifically mention healthcare, then the Federal government can't do anything about it.
For the census, here's what the Constitution says in Article I, Section 2, the section on the elction of members of Congress.
Representatives and direct Taxes shall be apportioned among the several States which may be included within this Union, according to their respective Numbers ...[this part, about slaves counting as three fifths of a person, was deleted by the Fourteenth Amendment]. The actual Enumeration shall be made within three Years after the first Meeting of the Congress of the United States, and within every subsequent Term of ten Years, in such Manner as they shall by Law direct.
They read this to mean the Census can only ask questions about how many people live at a given location. They can't ask their names. They can't write down the address. They can only count. The part about "in such Manner as [Congress] shall by Law direct" must mean something like whether the census counters have to count on their fingers or use paper.
The writers put Congress first in the Constitution--Article one. They must have thought Congress was pretty important, but the tenther line of argument doesn't leave the Congress much to do. They get to pass a budget, declare war, run the Post Office, determine whether census counters count on their fingers or use paper, and not much else.
Over the years, the same argument has been raised whenever the government is doing something that conservatives don't like or that provokes some kind of populist paranoia. In the thirties, conservatives and Republicans pulled out the Tenth Amendment argument to fight the New Deal and declaired that Roosevelt had ended constitutional rule and become a lawless tyrant. Southern politicians pulled out the Tenth Amendment argument during desegregation and declaired that Eisenhower had ended constitutional rule and become a lawless tyrant. When Medicare was being debated in the sixties, Ronald Reagan recorded a speech declairing that Johnson had ended constitutional rule and become a lawless tyrant. You get the idea.
These things sound ridiculous to most of us, but many Republicans are still fighting those battles. Newt Gingrich would like to repeal most of the social legislation of the sixties. Other Republicans are still trying to repeal the reforms and initiatives that Roosevelt used to end the Depression. When the sane among us try to laugh at the tenther argument by pointing out that, by their logic, Social Security, Medicare, veterans' hospitals, and the interstate highway system are unconstitutional, a frightning number of office-holding Republicans will look us in the eye and soberly answer that that, yes, they do think those things are unconstitutional.
Asking them about big government programs doesn't really expose the complete irresponsibility of the tenther argument. There are lots of things that the federal government is involved in, in one way or another, that are not specifically mentioned in the body of the Constitution. Among them:
- Predicting the weather
- Texas secceeding
- The Air Force
- Child pornography
- Preventing flooding on the Mississippi
- Helping people after flooding on the Mississippi
- The definition of marriage
- Public schools
- Illegal immigration
- Fighting forest fires
- The war on drugs
- Air traffic control
- People putting poison in our food
I think everyone can find something on that list that they think the government should not be involved with, but only the most over the edge libertarians would say they shouldn't be involved anything on that list. Somehow, mainstream Republicans have decided they need to pander to that fringe of the fringe.
One power that is specifically delegated to the government is to "fix the Standard of Weights and Measures." This means that while the government cannot do anything to fight child pornography, it can standardize shoe sizes. That's the original intent of the framers of the Constitution. That's how it should be.