Perhaps Sen. Charles Schumer should have taken the Quiet Car.

Here's what the New York Democrat reportedly was overheard saying the other day on Amtrak: "Even William Rehnquist is more moderate than they expected. The only one that resulted how they predicted [was] Scalia. So most of the time they've gotten their picks wrong, and that's what we want to do to them again."

Whether or not this quote from the Drudge Report is accurate, the sentiment is correct: The litany of conservative disappointments over Supreme Court appointments is a long one. Earl Warren. Harry Blackmun. John Paul Stevens. Anthony Kennedy and Sandra Day O'Connor, at least some of the time. David Souter (all of the time). President Eisenhower once said that appointing William Brennan to the bench was the biggest mistake of his presidency.

Why is that? Why do supposedly conservative justices so frequently disappoint the presidents who appoint them? And why is this disappointment so often a one-way ratchet, at least in modern times? After all, you rarely hear liberals complain about Stephen Breyer and Ruth Bader Ginsburg.

Some of our liberal friends might say, perhaps jokingly, that certain conservative justices have changed their leanings because they found wisdom and learned the error of their ways.

Those looking for a more plausible answer need to look deeper. Justices change their views because they are people, and, at the core, people want to fit in. They want to be respected and liked by those whose opinions they value. Supreme Court justices come to Washington from around the country -- Arizona, Illinois, New Hampshire, etc. -- and soon are immersed in, indeed overwhelmed by, the sea of Washington elites.

And let's be honest, today in Washington the elite norm is broadly liberal. Since the 1950s, America's major institutions -- the media, the world of arts and letters, the academy and the legal profession -- have become increasingly liberal. So if you want to fit in, in Washington, you usually fit in as a liberal.

The phenomenon is so common that frustrated conservatives even have a name for it. They call it the "Greenhouse Effect," named after The New York Times' influential court reporter, Linda Greenhouse, whose favorable coverage represents the mark of social acceptance in elite circles.

What Sen. Schumer instinctively understands is that there are really two types of conservatives. Some, like Sandra Day O'Connor, are conservative on particular issues, such as property rights. Their conservatism flows from cultural predilection. Others, though -- Antonin Scalia and Clarence Thomas come most readily to mind -- are philosophical conservatives, committed to interpreting the Constitution as written and resistant to the "politically correct" orthodoxy.

And that, at its core, is why conservatives are so focused on the next Supreme Court nominee: Because they know that only one type of conservative can resist the siren call of the Greenhouse Effect. They trust only conservatives of principle, those who have shown their ability to remain principled despite expediency. Conservatives like O'Connor, who are conservative by instinct alone without an underlying philosophy, often "grow" on the bench, to conservative consternation.

That's why conservatives want President Bush to nominate someone in the Scalia or Thomas mold. Conservatives have gotten their picks wrong before, and liberals want them to do it again.

You'll probably know who has carried the day as soon as the president names his selection. If it's an "instinctive" conservative, liberals will, once again, have lost the battle (when the nominee is confirmed) and won the war (when he or she "grows). It takes only a little time in the nation's capital for the Greenhouse Effect to work its magic.

Paul Rosenzweig is a senior fellow in the Heritage Foundation Center for Legal and Judicial Studies and a former Justice Department lawyer.