Elena Kagan did everything short of sign a covenant in her own blood to reassure gun-loving senators that she would never seek to undo a watershed Supreme Court decision extending the right to bear arms.

The Monday ruling set the tone for the repeated gun-related exchanges between Ms. Kagan and members of the Senate Judiciary Committee who, that very day, began hearings to evaluate her worthiness for the country's highest tribunal.

In McDonald v. Chicago, the court sided 5-to-4 in favour of a private citizen who contested the city's ban on handgun ownership. Justice John Paul Stevens, the retiring 90-year-old liberal lion whom Ms. Kagan would replace on the court, dissented with Chief Justice John Roberts, charging that the decision's consequences could be "destructive - quite literally - to our nation's communities and to our constitutional structure."

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But the Manhattan-bred Ms. Kagan, at 50, a self-confessed lifelong progressive Democrat, was unmoved. If she said it once, she said it a thousand times during the hearings that ended Thursday: McDonald is now "settled law" and she would never try to "mess" with it.

The powerful National Rifle Association wasn't buying it. On Thursday, it urged senators, who will vote later this month on Ms. Kagan's appointment by President Barack Obama, to reject the accomplished former Harvard Law School dean. The NRA said it found "no reason to trust her with Americans' firearms freedom."

The gun lobby's suspicion of Ms. Kagan is hardly misplaced. After all, Mr. Obama is an urban liberal. There is every reason to expect that he, like almost all presidents before him, would use his executive privileges to appoint judges to the top court who embody his judicial and political philosophies.

There's another reason, too. As a senator, Mr. Obama vehemently opposed George W. Bush's two nominees to the Supreme Court, Justice Samuel Alito and Justice Roberts. As President, he has repeatedly ripped into decisions made by the court's Roberts-led conservative majority, calling a January 5-to-4 ruling that struck down limits on corporate spending on elections "a victory for powerful interests that marshal their power every day in Washington to drown out the voices of everyday Americans."

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Judges, of course, are not avatars for those who name them. Some presidents have even openly regretted their choices. "I made two mistakes and both of them are sitting on the Supreme Court," Dwight Eisenhower famously quipped about Justices Earl Warren and William Brennan, both beacons of liberal activism.

But given Mr. Obama's antipathy toward Justice Roberts and his conservative cohorts, no one expects him to nominate judges who would rule like them. As a former part-time constitutional law professor himself, Mr. Obama is all too aware of the stakes involved.

Since Richard Nixon's election in 1968, Republican appointees to the court have outnumbered Democratic ones 13 to 4 (including Ms. Kagan). The result is a top court that has moved radically away from the liberal activism that characterized its civil rights, affirmative action and abortion decisions in the 1950s, 1960s and 1970s to one that favours a kind of conservative activism that treats corporations the same as ordinary citizens.

Mr. Obama has made no secret of his desire to change that. Ms. Kagan, currently Solicitor-General in his cabinet, and Justice Sonia Sotomayor, his first nominee, are an honest start. The death of liberal Justice Ruth Bader Ginsburg's husband a week ago is expected to accelerate her retirement. So, Mr. Obama could get a third nominee during his first term.

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But it is not until a conservative steps down - the first is likely to be Justice Antonin Scalia, 74 - that Mr. Obama can consequentially shift the balance on the court. He'll likely need to be a two-term president to do that. In the meantime, he is hoping Ms. Kagan's intellectual leadership will help sway Justice Anthony Kennedy, who currently sides most often with Justice Roberts, to join the other side on key decisions.

"I think it's certainly the case that President Obama is not in agreement with where the Roberts court is at on a number of issues. And he certainly would like to move the court away from that position," New York University law professor Richard Pildes offered in an interview. "How far he wants it to move, ideally, I don't think anyone possibly knows."

Nor is it clear how far, if at all, most Americans want him to move it.

Take gun rights. Not quite a year ago, during her own confirmation hearings, current Justice Sotomayor made similar statements to those of Ms. Kagan, referring to a 2008 gun decision, known as Heller v. District of Columbia, as "settled law." Yet this week, Ms. Sotomayor signed a dissenting opinion in McDonald that blatantly repudiated Heller, arguing that "the Framers did not write the Second Amendment in order to protect a private right of armed self defence."

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The court ruled in McDonald that the "right to bear arms" embodied in the U.S. Constitution's Second Amendment prevents state and local authorities, and not just the federal government, from coming between Americans and their guns. In Heller, it established for the first time that individual Americans have a fundamental right to own guns, dismissing the idea that the 18th century authors of the Constitution simply included the Second Amendment because militias needed to defend the largely rural new nation.

"I would be willing to bet my entire paycheque that, prior to Heller, an overwhelming majority of Americans would have said, of course, they have a constitutional right to some sort of personal weapons. So, Heller is very much a decision that recognized a long held constitutional value of the public," University of Georgia law professor Lori Ringhand said in an interview.

No one accused Justice Sotomayor of doing Mr. Obama's bidding in renouncing Heller. But it is more likely than not he agreed with her dissent. As a candidate for president, Mr. Obama officially hewed to the gun rights orthodoxy. But, for many Americans, his infamous off-the-cuff campaign trail comment about "bitter" small-town folk who "cling to guns or religion" betrayed his true colours.

It would be ludicrous to suggest the Supreme Court ever yields to public opinion. But if its decisions are to have lasting legitimacy, it cannot be entirely indifferent to how Americans view their country.

"The court should not and does not reflect what the public thinks today or this month or this year. That would make it duplicative of the elected branches and it would add no unique value," Prof. Ringhand explained. "But over the history of the U.S. Supreme Court, you will see that it rarely gets wildly out of whack with long-term public opinion. The Constitution lives in the sense that the judges make sure it is a constitution we can actually live with."

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Mr. Obama, by fate and by design, is presiding over the most fertile era of government activism since the New Deal - much of which was challenged all the way to the highest court. How the Supreme Court ultimately rules on the constitutionality of Mr. Obama's signature policies may depend on how many judges he gets to appoints.

Take Mr. Obama's health-care reform, which requires all Americans to buy health insurance starting in 2014. (Low- and middle-income Americans would receive generous government subsidies toward their purchase.) Virginia Solicitor-General Duncan Getchell, whose state is one of more than 20 challenging the new law, argued in court this week that it would be "unprecedented," "ahistorical" and "radical" to allow the federal government to require individuals to buy health insurance. It would not only impinge on individual freedoms, he said, it would violate states' rights to regulate insurance.

The health-care law is one element of Mr. Obama's "economic justice" agenda. Others, from tax reform to financial sector re-regulation, all involve empowering government and bridling free markets. To pass muster with the Supreme Court, it could also require some innovative legal thinking.

"The great economic and political challenges of our present decade - salvaging and fixing financial institutions, delivering health care, protecting the environment - have major constitutional dimensions. They require us to determine the limits of government power and the extent to which the state can impinge on collective and individual freedoms," Harvard Law School professor Noah Feldman wrote last month in The New York Times Magazine.

"A truly progressive constitutional project needs to go beyond simply upholding regulations challenged in court. It demands that the Supreme Court acknowledge the government's responsibility to protect our democracy from the harmful side effects of an all-powerful market."

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That would likely be a tough sell as long as Justice Roberts and his band rule the top bench. The survival of Mr. Obama's legislative legacy may depend on whether he remains President long enough to replace at least a couple of them.