Withholding federal highway funds from the state of California over its so-called “sanctuary state” policies would violate the Constitution and modern Supreme Court precedent.

In conservative circles it has become increasingly popular of late to call on Pres. Trump to ramp up pressure on California, with highway funds often playing the part of both carrot and stick.

For example, prominent conservative commentator and Students for Trump chairman Ryan Fournier recently tweeted, “If sanctuary states are unwilling to cooperate with federal law, President Trump should strip them of federal highway funds, which would have a great impact. Refer to South Dakota v. Dole.”

Citing the Supreme Court in South Dakota v. Dole (1987) is interesting choice, as that opinion would likely be one of the most prominent arguments against withdrawing federal highway funding from California.

This was a case where the Court considered how much pressure the federal government could put on the states. In 1984, Congress passed the National Minimum Drinking Age Act, withholding 5 percent of highway funds from states that didn’t raise their drinking age to 21. South Dakota refused and challenged the law in court.

Although the Supreme Court upheld the federal law, it came with some pretty specific limitations. One was a requirement that the amount of funding could be seen only as an “inducement,” and not be “so coercive as to pass the point at which pressure turns into compulsion.” In this case, a withholding of 5 percent was below this threshold as the Court noted this “constituted less than half of one percent of South Dakota’s budget at the time.”

Additionally, the Court has held that funding conditions on the States must be reasonably related. For example, taking away a law enforcement grant over a violation of an environmental regulation would likely be considered unrelated and unconstitutional.

All this was the foundation for something conservatives cheered as at least a partial victory for the Tenth Amendment and federalism in N.F.I.B. v. Sebelius (2012). Here, the Court cited Dole and held that provisions in the Affordable Care Act (ACA) requiring states to expand their Medicaid rolls or lose all Medicaid funding were “a gun to the head,” and unconstitutional.

In what might be seen as an interesting twist to this story, back in 2012, California and a dozen other blue states urged the Supreme Court to hold that thi