The president who has now nominated the unknown Judge David H. Souter to the nation's highest court is the man who cavalierly put Dan Quayle a heartbeat from the nation's highest political office. So it is only prudent to insist that Souter bear the burden of proving that he is not a Quayle -- not a cipher recklessly miscast for Bush's political convenience.

The convenience is as obvious as Souter's qualifications are, as yet, obscure.

Before the 1980 primaries, Bush said of the 1973 abortion ruling (Roe v. Wade): ''I happen to think it was right.'' His political situation, and hence his convictions, soon changed: he said the 1973 ruling was so wrong that the Constitution should be amended to change it. Republicans used the abortion issue to energize a particular constituency.

But public opinion has changed, so Republican convictions are changing. And at this awkward moment, Bush has had to nominate a justice.

Bush says he has not asked Souter what he thinks about Roe v. Wade, which means about the precedents that produced today's ''privacy right'' that produced Roe v. Wade. Bush either is not telling the truth or is telling a terrible truth about himself -- that he is willing to buy, and asking the Senate to buy, a pig in a poke.

Many senators will be under excruciating pressure to oppose Souter if he does not say he finds a right to abortion in the Constitution's ''privacy right'' (a right that is not mentioned in the Constitution). Some senators, and the White House, want to avoid the subject.

The White House waffle is: abortion is just one subject, not a litmus test. The real test is Souter's commitment not to legislate social preferences under the guise of construing the Constitution. But the most efficient way of ascertaining Souter's conception of, or aptitude for, the job is by asking what he thinks of the reasoning in Roe v. Wade.

Many senators were hoping for a nominee whose views on contentious issues are not, as Bork's were, well-known. But senators should not permit any nominee's reticence to survive the confirmation process.

It would be improper to ask Souter, ''What are your views about abortion?'' But it would be irresponsible not to ask, ''What do you think of the constitutional reasoning that resulted in creation of a privacy right that includes a right to abortion?'' It is as proper to ask a nominee's opinion of the reasoning behind Roe v. Wade as it would be to ask about the reasoning behind Brown v. Board of Education.

No nominee should be asked about pending cases. However, any nominee should be required to reveal the caliber of his or her mind by discussing the reasoning in important past cases, including those concerning capital punishment. That subject illustrates the anti-constitutional jurisprudence of Justice Brennan.

Because Brennan opposed capital punishment, he declared it unconstitutional. The Constitution in several provisions accepts such punishment, so Brennan said that such punishment has lately become ''cruel and unusual'' because of America's ''evolving standards of decency.'' But Brennan's supposedly empirical claim foundered on a fact: the public's notion of decency, expressed in polls and reflected in the laws state legislatures pass, favors capital punishment.

So Brennan argued that the Constitution defends human dignity, and justices can say what that requires, regardless of the original intention of constitutional language or today's democratic choices. Souter should be required to say what he thinks about Brennan's reasoning.

Souter should also be forced to be forthcoming on the subject of previous cases involving race-conscious government policies. Brennan wrote one of the crucial opinions affirming the constitutionality of today's racial spoils system. He helped transform the 1964 Civil Rights Act into a law creating special entitlements for government-approved minorities.

The act declares it unlawful for an employer to hire or fire on the basis of race. Brennan said Congress wanted to remove employment barriers to blacks, and race-based hiring is one way to do that, so Congress could not have meant quite what it said. Souter should say what he thinks of Brennan's reasoning.

Bush and his equally nervous and calculating minions argue, ludicrously, that it is fair to inquire about Souter's ''intellect'' and ''judicial temperament'' but, golly gracious, it would be ''inappropriate'' to seek Souter's opinion about the only matters that can definitively reveal his intellect and temperament. These matters include the court's arguments in the broad range of landmark cases that have reduced constitutional law to its current sorry condition.

As a man who prefers books to socializing, he is precisely what Washington needs more of. (We were at the same Oxford college 27 years ago.) It remains to be demonstrated that he is what the court needs.