This answer is what I got. I hope it helps.
The car full of teenagers crossed the state line on a summer evening in
1984. The four friends, all nineteen years of age, left their home state and
crossed into South Dakota, because in South Dakota they could legally buy beer.
As their car cruised along the interstate, lawmakers in Washington D.C. were
writing the National Minimum Drinking Age Amendment. States that did not raise
their drinking age to twenty-one would receive 5% less money for their
highways.
South Dakota challenged the conditional highway funding, saying that it was
an over-reach of Congress’s power as well as a violation of the Twenty-First
Amendment, which gave states the exclusive power to regulate alcohol. The
Supreme Court disagreed, saying Congress can “indirectly” encourage uniformity
among the states through spending power, provided that the condition is
“reasonably related” to the purpose of the funding, in pursuit of the “general
welfare,” clearly defined, related to a national program or interest, and does
not force states to do things that would be unconstitutional.
Calling the law merely “relatively mild encouragement” to the States to
enact higher minimum drinking ages, the Court found the federal law
constitutional. “We find this legislative effort within constitutional bounds
even if Congress may not regulate drinking ages directly.” Looking at findings
that the different state drinking ages created “an incentive to drink and
drive,” the Court found the law consistent with the goal of promoting the
general welfare of the nation. Congress was able to effectively establish a
national minimum drinking age, even though this is not a power delegated to it
in Article I.
In her dissent, Justice O’Connor argued that the funding condition was
unconstitutional. It would not markedly increase safety because “teenagers pose
only a small part of the drunken driving problem in this Nation.” She concluded
that establishing a national minimum drinking age had “nothing to do” with
highway funding. Rather than “determining how federal highway money shall be spent,
it is a regulation determining who shall be able to drink liquor.”