A decision handed down by the Supreme Court earlier in December may have disturbing implications for Fourth Amendment rights. But Heien v. North Carolina is notable for another reason: it marks the emergence of Sonia Sotomayor, President Obama's first Supreme Court nominee, as the strongest voice for civil liberties on the court.

There are many who may be surprised by the clout she has amassed in just a few short years. It's worth remembering the remarkable condescension — in some cases tinted with sexism and racism — that greeted her nomination.

Sotomayor's formal credentials were essentially identical to President George W. Bush's second nominee, Samuel Alito: Princeton undergraduate degree, Ivy League law degree, prosecutor, long and distinguished career on the federal judiciary. But while nobody questioned whether Alito was qualified (as opposed to ideologically suitable) for the bench, many conservative pundits asserted that Sotomayor lacked the the basic qualifications for the job. Some even compared Sotomayor to Harriet Miers, the failed Bush nominee who was very similar to Sotomayor except for her lack of elite educational credentials, judicial experience, or basic knowledge of constitutional law. (Tellingly, they do share one thing in common: their sex.)

Nor did these criticisms come solely from the right. Like many others, I'm saddened by the recent mass resignations at The New Republic. But I can't forget that it published a disgraceful hit job on Sotomayor by its legal affairs editor Jeffrey Rosen. The article's anonymous smears, which attacked her temperament and intelligence, were short on substantiation and long on the sexist stereotypes that have been used to characterize women on the bench for years.

As Joan Biskupic demonstrates in Breaking In, her fine new book about Sotomayor, after a somewhat tentative beginning Sotomayor has become a very important liberal voice on the court. Her draft opinions in the Fisher affirmative action case, for example, may well have prevented the court from ruling virtually all affirmative action in higher education unconstitutional. She ultimately withdrew the opinions, after Justice Kennedy narrowed his opinions in an apparent response to Sotomayor.

The recent Fourth Amendment ruling, while not as important, is another case in point.

In Heien, the court ruled that a police search of a car that ultimately found cocaine was "reasonable" under the Fourth Amendment, although the single broken brake light the police used as a pretext to stop the car was legal under North Carolina law. Sotomayor was the only dissenter. Two of the liberal justices who voted with the majority — Justices Ginsburg and Kagan — emphasized the unusual facts of the case: the North Carolina courts had not definitively ruled that driving with only one broken brake light was legal until after the arrest.

But as Sotomayor noted in her dissent, the ruling has consequences that extend beyond this case. The vague standard created by the majority — that a search based on a misunderstanding of the law is permissible under the Fourth Amendment as long as the police officer's misinterpretation of the law was "reasonable" — has the potential to create a substantial amount of mischief. Lower courts are likely to use the standard to give the policy more leeway to conduct searches based on legal errors, a problem that is exacerbated by the fact that the Supreme Court did not provide adequate guidance to the lower courts.

In addition to her dissent, as Slate's Dahlia Lithwick observes, at oral argument Sotomayor was the only justice to raise the disturbing, broader issues lurking in the background of the case. A Hispanic man had his car pulled over and extensively searched after being followed for the utterly banal behavior of having a 10-and-2 hand position on his steering wheel and "looking straight ahead." Even if the police did find a legitimate, albeit trivial, violation of the law to justify pulling the suspect over, the search raises very serious Fourth Amendment problems. The court has refused to take these issues seriously, and it can be expected that Sotomayor, the court's first Latina justice, will continue to urge her colleagues to do so.

Nor is this the first time that Sotomayor has made a major contribution to the court's civil liberties jurisprudence. In a solo concurrence in a case involving GPS tracking, Sotomayor emphasized that the court's assumption that individuals almost never have an expectation of privacy with respect to data shared with third parties has become an anachronism in an age in which people routinely save large amounts of personal information on their phones. She also underscored the extent to which government surveillance "chills associational and expressive freedoms." Her concurrence has already proven to be highly and deservedly influential.

Combined with her pathbreaking writing on racial equality, Justice Sotomayor's civil liberties jurisprudence has proven her detractors very wrong. We can only hope that more of her views will be expressed in majority opinions, rather than in lone dissents and concurrences.