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.”