When Ronald Reagan nominated Arizona's Sandra Day O'Connor to the Supreme Court in 1981, conservatives were nervous because little was known about her. Reagan assured religious conservatives they had nothing to fear.

Reagan told the Rev. Jerry Falwell he had spoken to her about abortion, which was the main concern of religious conservatives, and found her to be "OK" on that issue. Reagan assured Falwell and company they would not be disappointed.

I was vice president of Falwell's Moral Majority at the time and went on ABC's "Nightline" to express my reservations that conservatives might not like what they were getting. What I had seen of O'Connor's record did not persuade me she would favor restricting abortion.

I was right, and Reagan was wrong. Conservatives were disappointed. O'Connor has been the key vote upholding the extra-constitutional ruling known as Roe v. Wade.

There would be other justices named by Republican presidents who also were disappointments. Anthony Kennedy was chosen by Reagan after his administration misjudged the intensity of opposition to Judge Robert Bork. Kennedy has been a disaster on abortion and religious issues.

David Souter was nominated by the current president's father after similar assurances by then-White House chief of staff John Sununu that Souter would be OK on issues about which conservatives cared. He wasn't. Souter has been as liberal as any justice in recent memory.

Despite her thin legislative and judicial record in Arizona, there were hints about O'Connor's legal philosophy from Eleanor Smeal, then-president of the National Organization for Women. Last week, Smeal recalled she endorsed O'Connor's nomination before the Senate Judiciary Committee because "I knew then that O'Connor, although a conservative voice, would be one who would not permit the elimination of women's fundamental rights, including the right to privacy."

Instead of seeing this as a red flag, most conservatives held their tongues. They wanted to maintain access to Reagan.

This history is what makes conservatives nervous about the choice President George W. Bush will make, especially when he speaks of symbolism and the potential nomination of the first Hispanic justice, possibly Attorney General Alberto Gonzales. Reagan tried symbolism by naming the first woman, but he lost substance.

We hear this President Bush has learned a lot from the mistakes of his father. Does this include naming a justice that reflects his often-stated views about wanting someone on the bench who doesn't make law but rather upholds the Constitution? We are about to find out.

More than campaign promises, Bush's first choice of a Supreme Court justice will reveal his core beliefs. He has repeatedly said he wants someone in the model of Clarence Thomas and Antonin Scalia. These are men who have lived up to the noble objective of faithfully interpreting the Constitution instead of unfaithfully reading into it their personal judicial preferences.

An unnamed senior administration official told The New York Times, "The president is going to pick someone who is a true constructionist and who is correct in interpreting the law."

The left already is mobilizing to smear whoever is selected as an extremist, an out-of-mainstream nominee who will recreate back-alley abortions and resurrect the Dark Ages.

Conservatives say they have learned from previous court battles and are not going to be fooled again. They will look beyond assurances that a nominee is OK and examine the substance of that nominee's record and philosophy. Nothing but delivery on the president's promise will satisfy them.

This is the big one, the main event. If the president does not nominate someone who measures up to his often-stated view of the court and the Constitution, he can forget about conservative support for anything he wants to do during the rest of his term. Even if he names someone who eventually is rejected by the Senate, he will get significant support from conservatives and momentum for nominating another conservative.

Cal Thomas is a nationally syndicated columnist and a host on Fox News Channel. Readers may leave e-mail at www.calthomas.com . His column appears on Tuesday and Thursday.