Many hope, and many others fear, that President Obama will choose a crusading liberal activist to energize the Supreme Court’s progressive wing.

Such an appointee might push to expand racial preferences, abortion rights, and especially welfare rights for poor people; to strike down the law barring openly gay people from the military; to recognize gay marriage (which Obama has opposed); to end the death penalty and curtail gun rights (both of which he has supported); to free Guantanamo detainees unless they can be convicted of crimes (which would reject Obama’s policy); and much more.

The preceding parentheticals suggest some of the reasons I’m cautiously betting that Obama will choose a moderate liberal who believes in judicial restraint. By this I mean deference to elected officials unless they violate clear constitutional commands or show gross irresponsibility. The lack of such restraint is what I mean by "judicial activism."

A restrained liberal justice might, for example, hope for legislative recognition of same-sex marriage (as do I) but decline to rewrite the Constitution to override the democratic process on the issue by judicial decree.

This is not to suggest that the president will pick a centrist, let alone a conservative. Filling moderately left-of-center Justice David Souter’s seat with anyone seen as more centrist would be a stunning abandonment of Obama’s campaign stance that would infuriate his liberal base.

But nominating a crusading liberal activist could seriously jeopardize the president’s own best interests, in terms of policy as well as politics. And although some of Obama’s past statements are seen by critics as a formula for judicial activism, he has also shown awareness of its perils.

As a matter of policy, consider Obama’s most important responsibility: protecting our national security from jihadist terrorism and other threats.

As I have noted briefly, the intersection of law and national security will provide the most consequential cluster of issues that the Supreme Court will consider over the next decade or more. Obama surely understands that the Court’s response to his national security policies will be more important by far to the success of his presidency than any decisions on abortion, race, religion, gay rights, crime, or free speech.

Obama’s national security policies are already under relentless attack from leading advocates of liberal judicial activism, such as the ACLU. Indeed, most (or at least many) lawyers and scholars who favor a liberal activist approach on social issues also tend to support relatively broad judicial power to overrule the president on national security.

The justifiable rejection of President Bush’s wildly excessive claims of near-dictatorial war powers by the five more-liberal justices — including Souter and swing-voting centrist Anthony Kennedy — has a downside for Obama. The justices, followed by the lower courts, have now asserted far more power than ever before to oversee and second-guess presidential decisions about national security.

Meanwhile, in moving from campaign mode to the presidency, Obama has had many reasons to worry about such judicial second-guessing. One federal District judge has rejected the administration’s claim that it can detain suspected jihadist fighters captured outside Afghanistan at Bagram Air Base without judicial oversight. A federal Appeals Court has rejected the White House’s efforts to use the "state secrets" doctrine to block lawsuits by former detainees who claim they were tortured.

Still other lawsuits demand the release of any detainees in the war on terrorism who cannot be convicted of crimes, and publication of classified CIA documents that Obama would rather keep under wraps. A reported, perhaps tentative plan by administration officials to use "military commissions" instead of ordinary courts to try some of the detainees for war crimes would surely bring more legal attacks. And for the foreseeable future, squadrons of liberal lawyers will be suing a range of companies for cooperating with the president on matters such as wiretaps, "renditions" of suspected terrorists to other countries, and other actions deemed by Obama to be vital to national security.

The more the courts smile on such lawsuits, the harder it will be for the president to protect the country. Indeed, some human-rights and civil-liberties activists have done their best to hamstring virtually all of the surveillance, search-and-seizure, detention, and related powers on which the government depends to find and disable suspected terrorists.

It’s unclear how Obama would fare in such cases with the current Court. But he would surely run the risk of seeing some of his key security policies overturned if he were to choose someone who turns out to be more aggressive than Souter in curbing presidential war powers.

This logic argues for a nominee likely to be relatively deferential to presidential power. But not that many prominent lawyers combine that kind of deference with a liberal activist approach to social issues such as abortion, race, and gay rights.

As a political matter, Obama would benefit considerably from a quick, relatively easy confirmation proceeding. The lopsidedly Democratic Senate would eventually confirm just about anyone Obama might choose. But Republicans will be looking to drag out the Senate confirmation process and to portray the nominee as a liberal activist determined to override the will of the voters.

Such efforts will be more successful if the attacks ring true in light of the nominee’s record. This could burn up precious floor time that Obama needs to push through his hugely ambitious policy agenda. It could also endanger some congressional Democrats in 2010 elections and thereafter.

Now consider what Obama has said about judicial activism. Conservatives point accusingly to his stated preference for judges with "the empathy to understand what it’s like to be poor or African-American or gay or disabled or old." Such critics see this as an endorsement of judicial activism (as well as of judicial bias). But Obama, an accomplished constitutional scholar in his own right, has also displayed understanding of the case for judicial restraint.

In his 2006 book, The Audacity of Hope, Obama spoke favorably of "moderate nominees who can garner some measure of bipartisan support." While that can be seen as political positioning, Obama also cautioned against bold judicial social engineering in a 2001 radio interview in which he spoke approvingly of "redistribution of wealth."

John McCain’s presidential campaign jumped on that phrase, suggesting that Obama had endorsed judicial redistribution of wealth. This was false. In fact, Obama’s complaint was that "the civil-rights movement became so court-focused" as to neglect "political and organizing activities." He added a note of caution against seeing the Warren Court’s extraordinarily bold attack on an entrenched system of racial oppression as a precedent for a judge-led war on poverty.

"The Constitution is a document of negative liberties," Obama said in that interview. "[It] says what [government] can’t do to you … but it doesn’t say what [government] must do on your behalf…. I am not optimistic about bringing about major redistributive change through the courts. You know, the institution just isn’t structured that way…. The court is not very good at it and politically it is hard to legitimize opinions from the court in that regard…. So I think that although you can craft theoretical justifications for … bringing about economic change through the courts, I think that as a practical matter that our institutions are just poorly equipped to do it."

Obama also suggested then that "unelected judiciaries making laws in what is supposed to be a democracy" undermined the legitimacy of their decisions. Former colleagues at the University of Chicago Law School, where he taught, have described the president as skeptical of the courts’ ability to improve on the political process in resolving big public policy issues.

All of this suggests that Obama’s nominee may disappoint believers in crusading judicial activism. It also suggests that the president may have a very hard time finding his ideal justice.

Even if he can make a list of candidates who are liberal — but not activist — on social issues and not so liberal on curbing presidential war powers, he would feel pressure to cross off any who happen to be white males, in order to diversify the Court’s demographic profile. He also wants to diversify the professional profile of a Court now consisting of nine federal Appeals Court alumni, eight of whom studied law at Harvard or Yale.

What Obama needs, in short, is an intellectually stellar, not-too-old, Hispanic woman lawyer with empathy for the powerless; views on social issues that are predictably liberal but not so activist as to inflame the Right; views on presidential war powers that are predictably deferential but not so much so as to inflame the Left; broad real-world experience; and, of course, rapport with Obama.

No such human being exists, I suspect. I also suspect that the president may come to see the opportunity to choose a new justice as a lot less fun than a law professor might imagine.

This article appeared in the Saturday, May 9, 2009 edition of National Journal.