Because the basic jurisprudential claim that original understanding is the only legitimate method of interpretation is overstated, originalists usually make a second, more pragmatic argument: A living Constitution offers insufficient constraints on judicial power. The irony of this charge is that in practice originalism doesn't provide any greater constraint. As we've seen, originalist judges pick and choose when to invoke original understanding and when to rely on existing precedents they like. Justices Scalia and Thomas, for example, haven't acknowledged in their opinions that the Congress that passed the 14th Amendment also engaged in affirmative action for blacks; both have pushed hard for ever-greater protection of commercial speech without any evidence of the original understandings of 1791. If we want examples of judges just making stuff up to satisfy their personal predilections, so-called originalist judges offer plenty of examples.
Originalists are right: Constraining judges is important. But originalists are looking in the wrong place. Lower courts are strongly constrained by previous precedent. Constraints on the Supreme Court come from two sources—the professional legal culture and constitutional structure.
Legal culture demands that arguments depend on the familiar categories of text, history, precedent, and structure. These modalities allow considerable leeway, but they also genuinely constrain. Even though the Supreme Court chooses the most controversial cases, many, if not most, of its decisions are unanimous or include both liberals and conservatives in the majority.
The second, and more important, background constraint on the Supreme Court comes from the constitutional structures of American government. Because the court is a multimember body, centrist judges in each generation, like Lewis Powell or Sandra Day O'Connor, determine the path of doctrine, especially in the most controversial areas. In addition, new Supreme Court appointments tend to respond to the vector sum of the political forces at play at the time of confirmation. In fact, political scientists have shown repeatedly that the Supreme Court never strays too far too long from the center of the national political coalition, and when it does, new appointments tend to push it back in line. The Supreme Court held out against Franklin Roosevelt's New Deal for a few years but eventually gave in. The New Deal settlement, which Justice Scalia himself believes in, came from overwhelming public sentiment in the '20s and '30s that the Constitution had to be interpreted in light of the needs of the time; that ours was a living Constitution.
Since the nation began, critics of the Supreme Court have argued that judges are about to take over the country and rule by fiat. It hasn't happened yet. What critics don't recognize is this: Checks and balances built into the system guarantee that the court rarely opposes the national political coalition for long, and it usually cooperates with it. It's a good bet that people who complain the loudest about the court being countermajoritarian represent at most a regional majority, not a national one. People in the political center usually get pretty much what they want.
And that brings us to the real secret of why we have a living Constitution. In the long run, the Supreme Court has helped secure greater protection for civil rights and civil liberties not because judges are smarter or nobler, but because the American people have demanded it. When social movements like the civil rights movement or the feminist movement convince the center of the country that their claims are just, the court usually comes around. Sometimes it gets ahead of the center of public opinion, and sometimes it's a bit behind. But in the long run it reflects the national mood about the basic rights Americans believe they deserve. The great engine of constitutional evolution has not been judges who think they know better than the American people. It has been the evolving views of the American people themselves about what rights and liberties they regard as most important to them.
Rather than a set of shackles designed by long-dead slave-owners, the framers bequeathed to us a Constitution that could adapt to the needs and aspirations of each succeeding generation. Their faith in the possibilities of the future, and our enterprise in realizing that future, have made us the great and free nation we are today.