This argument suggests the method and appeal of democratic constitutionalism. While a Warren Court liberal might counsel the Supreme Court to leap ahead of public opinion and provide constitutional protections for gay marriage today, and while a minimalist might urge state and federal courts to wait until public opinion has shifted decisively, a democratic constitutionalist would embrace bold state court decisions but hold back at the federal level. In tandem with gay-marriage activists, in other words, state courts can play a role in building a national consensus for gay marriage that the U.S. Supreme Court may eventually recognize, protect and enshrine in constitutional terms.

The democratic constitutionalists also offer a different historical account than do minimalists or Warren Court liberals of how constitutional rights have actually been expanded in the past — a history that parallels their own prescription for how constitutional rights ought to be expanded. In a forthcoming book, “The Will of the People,” Barry Friedman, a law professor at New York University, tells the story of how the “give-and-take between the courts and the people” has shaped constitutional law from the founding fathers’ era to the recent past and present.

Consider the example of women’s equality. As late as 1961, Friedman notes, the Supreme Court held that the government was permitted to enforce traditional sex roles — excluding women from jury service, for example, on the grounds that “woman is still regarded as the center of home and family life.” But of course the country’s mood was shifting. The new wave of feminism and the entry of women into the labor market were changing the popular understanding of women’s equality. In 1972, Congress sent the Equal Rights Amendment to the states, and for a time ratification seemed likely. In 1973 a plurality of the Supreme Court argued that sex discrimination was entitled to the same rigorous constitutional scrutiny as race discrimination. But the E.R.A. was never ratified, and the Supreme Court didn’t go as far as the plurality suggested it might. In 1976, the court ruled that sex discrimination deserved only slightly less constitutional scrutiny than racial discrimination — a principle that even the E.R.A.’s opponents claimed to accept.

To a Scalia-style strict constructionist, the Supreme Court decisions forbidding sex discrimination were presumably mistakes that should be at most grudgingly tolerated, because they clashed with the original meaning of the Constitution. To a Clinton-era minimalist, the court was presumably wrong to leap ahead of popular understandings of women’s rights and should have waited for the ratification of the E.R.A. To a Warren Court liberal, the court presumably should have held in the early ’60s that race and sex discrimination were entitled to the same rigorous constitutional protection. But to a democratic constitutionalist, the court was right to be cautious and bold at the same time — waiting until the E.R.A. was proposed but not waiting for its ratification, on the theory that the court’s decisions could help push the country in a direction it was already moving and could help solidify the moral and legal cases for why sex discrimination was wrong.

For critics, the democratic constitutionalist’s metaphor of “dialogue” is likely to seem vague and unsatisfying, a malleable abstraction that doesn’t give a clear sense of which larger principles judges are supposed to honor, or when boldness is preferable to caution. Conservatives will certainly charge that the new liberals are guilty of the same sins as the old liberals: urging judges to disguise their policy preferences with ad hoc legal “principles.” The democratic constitutionalists respond that, although their method can’t be reduced to scientific precision, it’s far more respectful of democratic debate — far more willing to let contested issues be solved by the political branches than by judges — than the leading liberal or conservative alternatives.

This summer, Obama will have a chance to appoint his first justice to the Supreme Court, and in the coming years, he may have the opportunity to appoint others. How likely might his choices be to embrace democratic constitutionalism? Many possible candidates can be seen as sympathetic to this emerging school of thought. His first nominee, Judge Sonia Sotomayor, as well as another judge often mentioned as a possible future candidate, Judge Diane Wood, have each publicly acknowledged that courts can shape progressive values by working as partners with legislatures and political activists. In a 2004 lecture at New York University, “Our 18th-Century Constitution in a 21st-Century World,” Wood sounded very much like a democratic constitutionalist when she described the “interactive process” of constitutional change “that occurs through dialogue among the Supreme Court, the lower courts, legal scholars and society at large.” Similarly, Sotomayor, whose past statements will now be heavily scrutinized, in a 1996 article that she co-wrote, based on a speech she herself gave at Suffolk University Law School in Boston, described “the law as a dynamic system,” arguing approvingly that “change — sometimes radical change — can and does occur in a legal system that serves a society whose social policy itself changes.”

Obama has said that he wants justices who understand that “justice isn’t about some abstract legal theory or footnote in a casebook; it is also about how our laws affect the daily realities of people’s lives.” Perhaps he was saying that rather than define our rights in an ivory tower, the most successful justices recognize that their decisions are part of a dialogue with real people — starting at the grass-roots level. Or perhaps Obama’s constitutional vision is less fully developed than I’ve suggested. It’s clear, though, that Obama is trying to impose not a hard-and-fast manifesto for judicial liberalism but a sensibility and a work in progress that seek to synthesize previous legal thinking into something cohesive and new. If his nominees prove as ambitious as the president himself, they may well transform the shape of liberalism.