That Republic is manifest in the Constitution. And that Constitution was created by the People.
I think you ignored the key question about this topic: forget about the theory for a moment, but what are the feds going to do?
Let's say that the Feds try to withhold highway funding--ok, Montana decides to play hardball and just simply refuses to remit the taxes to the federal government in the firstplace.
It comes down to sending in troops, and I just don't see that happening. If Montana doesn't want to play ball, it's pretty tough to make it. The whole system that we've got set up here pretty much depends on the states going along for the ride.
And it granted and enumerated the powers of the Federal Government. Those powers were fixed at the time they were transferred, and remain unchanged until altered by amendment.
"I write separately only to express my view that the very notion of a ‘substantial effects’ test under the Commerce Clause is inconsistent with the original understanding of Congress’ powers and with this Court’s early Commerce Clause cases. By continuing to apply this rootless and malleable standard, however circumscribed, the Court has encouraged the Federal Government to persist in its view that the Commerce Clause has virtually no limits. Until this Court replaces its existing Commerce Clause jurisprudence with a standard more consistent with the original understanding, we will continue to see Congress appropriating state police powers under the guise of regulating commerce."
- Justice Clarence Thomas
For a like reason, I made no reference to the "power to regulate commerce among the several States." I always foresaw that difficulties might be started in relation to that power which could not be fully explained without recurring to views of it, which, however just, might give birth to specious though unsound objections. Being in the same terms with the power over foreign commerce, the same extent, if taken literally, would belong to it. Yet it is very certain that it grew out of the abuse of the power by the importing States in taxing the non-importing, and was intended as a negative and preventive provision against injustice among the States themselves, rather than as a power to be used for the positive purposes of the General Government, in which alone, however, the remedial power could be lodged.
- James Madison to Joseph C. Cabell
13 Feb. 1829
Letters 4:14--15