At both ends of the ideological spectrum, politicians, activists, journalists, and academics like to stress how big a change the next Supreme Court justice could make in the course of the law. The appointment will, says the conventional wisdom, be among President Obama’s most important legacies.

Many also stress how far to the right (say liberals) or left (say conservatives) of center the Supreme Court has been in recent years, the better to dramatize the need to correct the perceived imbalance.

And the dominant media image has been of "the conservative Court" (recent articles in The Washington Post), or "the Supreme Court’s conservative majority" (New York Times editorials), or a Court "as conservative as it’s been in nearly a century" (Newsweek commentary by my friend Dahlia Lithwick).

All this brings to mind three contrarian theses.

First, it simply won’t make much difference in the next five or so years — if ever — whom Obama picks from the lists of moderately liberal, extremely liberal, and just plain liberal candidates leaked by the White House.

Indeed, I can’t think of a single case or issue that would foreseeably be decided differently depending on whether the nominee turns out to be the most or the least liberal of those under serious consideration.

The Court is by nature quite stable. Imagine, for example, that Obama nominated and the Senate confirmed a person more liberal than either John Paul Stevens or any other current justice. No matter how passionate, or how brilliant, or how persuasive, he or she could move the law no further than at least four others were willing to go. And given the justices’ fierce independence, it’s hard to imagine any of them lurching leftward at the urging of the new kid on the block.

The eventual impact of the next justice’s ideology will depend on unpredictable developments, including how many allies he or she might gain from future appointments and how his or her own views might evolve, both on today’s big issues and on issues that will emerge later.

Polls suggest lopsided public disapproval of Supreme Court decisions on Guantanamo detainees.

Indeed, history suggests that an appointee is likely to make a dramatic difference only when three ingredients are present: 1) The president is liberal and the outgoing justice is conservative, or vice versa; 2) the Court is very closely divided along liberal-conservative lines; and 3) the president can get a strong proponent of his own ideology through the Senate.

The first ingredient is not present now because Obama and Stevens are both liberal.

The first two ingredients were present in 1987, with the retirement of moderate Justice Lewis Powell, who had been the balance-tipping vote. President Reagan named Robert Bork, who was poised to provide the fifth vote to overrule Roe v. Wade and other major liberal precedents. But the Senate rejected him.

That led to the appointment of Justice Anthony Kennedy, who now occupies the same balance-tipping position that Powell once did. Kennedy was far less conservative than Bork, as he showed by reaffirming Roe in 1992 (with some trimming). Kennedy also proved to be more liberal than Powell on some big issues. That helps explain why during Kennedy’s time the Court has moved to the left — although you’d hardly know it from most media portrayals — on national security, gay rights, the death penalty, and church-state issues.

The only time in the past 35 years that all three ingredients for a balance-tipping appointment have been present was 2005. The retirement of moderate-liberal Justice Sandra Day O’Connor allowed President Bush to move the Court to the right — but not so far as to fit the "conservative Court" caricature — by nominating the solidly conservative Samuel Alito, who was confirmed in 2006.

(Bush’s 2005 appointment of John Roberts to succeed William Rehnquist as chief justice did not substantially change the Court’s balance.)

This brings me to my second contrarian thesis: Despite complaints by both liberal and conservative critics, the Court has not strayed far from mainstream public opinion over the past 35 years.

The need to get the support of both the president and the Senate tends to screen out candidates with extreme views. And the Supreme Court does follow the election returns, as humorist Finley Peter Dunne’s Mr. Dooley observed. Polls, too.

Indeed, no decision since Roe in 1973 — not even Bush v. Gore — has produced a lasting public backlash. And that one was a surprise to the justices.

My third contrarian thesis is that although Alito did make the Court more conservative on some big issues, we have not had a consistently "conservative Court" since 1937. Indeed, since the 1970s, the Court has strayed more often to the left than to the right of center of public opinion. And it remains as liberal as ever on some big issues, if only by a one-vote margin.

Here’s a brief issue-by-issue analysis, updating a more detailed commentary that I posted last June 16 on National Journal’s Ninth Justice blog.

• National security. In three big cases involving Guantanamo detainees since 2004, the four liberals plus Kennedy (and O’Connor in the 2004 case) have pushed the judiciary deeper into second-guessing the political branches’ national security policies than ever before, by sweeping aside both presidential orders and a major 2006 act of Congress. Polls suggest lopsided public disapproval.

• Abortion. Polls have shown for many years that although the public does not want Roe v. Wade overruled, majorities say that abortion should be legal in, at most, "only a few circumstances" and support restrictions that the four liberals and Kennedy have struck down. These include banning abortions in the second and third trimesters and requiring spousal notification. Kennedy did join the four conservatives in 2007 in upholding a federal ban on "partial-birth" abortion that O’Connor would have struck down, a decision that enjoyed overwhelming public support.

• Religion. Polls have consistently shown strong approval of the nondenominational, nonparticipatory types of school prayer outside the classroom setting that the liberals plus Kennedy and O’Connor struck down over conservative dissents in 1992 and 2000.

• Death penalty. The four liberals and Kennedy have banned the death penalty for murderers who were mentally disabled or younger than 18 and for child-rapists. These decisions — the last of which candidate Barack Obama denounced in 2008 — put the justices somewhat to the left of public opinion.

• Gay rights. Although a solid majority of the public believes that sexual relations between consenting gay adults should be legal, polls showed that many thought the four liberals and Kennedy had gone too far, too fast when they made gay sex a constitutional right in 2003. Roberts and Alito have yet to face a gay-rights case.

• Gun rights. The June 2008 decision by the Court’s conservatives and Kennedy striking down the District of Columbia’s complete ban on handguns was an unprecedented interpretation of the Second Amendment. But it was also consistent with the overwhelming public support for an individual right to keep and bear arms — and was applauded by Obama.

• Federal power. Conservatives plan to argue during the coming confirmation proceedings that the health care overhaul’s mandate to buy insurance or pay a penalty tax exceeds Congress’s constitutional powers. It is not yet clear whether Alito and Roberts would be more receptive to such claims than O’Connor and Rehnquist.

• Race. This is the biggest issue on which Alito appears to have made the Court markedly more conservative than it had been. In a 2007 decision striking down race-based student assignments — which would almost certainly have gone the other way had O’Connor stayed on — the four conservatives came close to adopting an absolutist "colorblind Constitution" stance that would doom racial preferences. Kennedy didn’t go as far, but has generally opposed racial preferences — as has the public, by wide margins.

• Campaign finance regulation. In striking down on January 21 all limits on independent election spending by corporations and (apparently) unions, the five-justice majority moved dramatically to the right not only of its own precedents but also of public opinion, which was overwhelmingly negative. This was big. Whether it foreshadows bigger things to come is unclear.

Democrats are trying to leverage public disapproval of this and other decisions into a populist backlash against justices who "always seems to side with the big corporate interests against the average American," in the words of Senate Judiciary Committee Chairman Patrick Leahy. Lest mere exaggeration fall short of galvanizing the public, Leahy, Obama, and many in the media have added a large dose of distortion. (For more NJ coverage of this issue, see, for example, "Does the Ledbetter Law Benefit Workers, or Lawyers?")

The reality is that although Alito has moved the law to the right on some issues, the Court still has five solid liberal votes on others, thanks to Kennedy’s ideological eclecticism. The same will almost certainly be true when the dust clears after this summer’s confirmation vote.

This article appeared in the Saturday, April 17, 2010 edition of National Journal.