WASHINGTON — President Obama now says he cannot imagine a constitutional justification that would lead to a court upholding a ban on allowing same-sex couples to marry, the furthest statement in support of same-sex couples' marriage rights made by the president.

In an interview with ABC News' George Stephanopoulos, Obama said a state would need a "good justification" for treating gay and lesbian couples differently than straight couples in order for a court to find a ban on same-sex marriages to be constitutional.

But when Stephanopoulos asked Obama if he could imagine such a justification, Obama replied, "I can't, personally. I cannot."

The Supreme Court will be hearing a challenge to one such ban, California's Proposition 8, on March 26. The Obama administration weighed in to support the effort to have Proposition 8 found unconstitutional, but the brief did not directly state that the administration viewed all such bans as unconstitutional.

Here's the transcript of that portion of the interview provided by ABC News:

GEORGE STEPHANOPOULOS: Let me ask you a question about gay marriage. When Robin was here last spring, you came out in favor of gay marriage. But you also said at the time that you wanted it to be a state-by-state–



PRESIDENT BARACK OBAMA: Yeah. Yeah.



GEORGE STEPHANOPOULOS: –issue, it would be a mistake to nationalize it. Do you still believe that, or do you now believe that gay marriage is a right guaranteed to all Americans by the Constitution?



PRESIDENT BARACK OBAMA: Well, I've gotta tell you that– in terms of practical politics, what I've seen is a healthy debate taking place state by state, and not every state has the exact same attitudes and cultural mores. And I– you know, my thinking was that this is traditionally a state issue and– that it will work itself out.



On the other hand– what I also believe is that the core principle that people don't get discriminated against– that's one of our core values. And it's in our constitution. It's in– the– you know, 14th Amendment and the Equal Protection Clause. And– from a legal perspective, the– the– the bottom line is, is that gays have historically been discriminated against and I do think that courts have to apply what's called heightened scrutiny, where they take a careful look. If there's any reason for– gays and lesbians to be treated differently, boy, the government better–



GEORGE STEPHANOPOULOS: So banning gay marriage–



PRESIDENT BARACK OBAMA: –have a really good–



GEORGE STEPHANOPOULOS: –is discrimination?



PRESIDENT BARACK OBAMA: Well, what I– what I believe is that– if– if the states don't have a good justification for it, then it probably doesn't stand up to constitutional muster



GEORGE STEPHANOPOULOS: Can you imagine one?



PRESIDENT BARACK OBAMA: So– well, I can't, personally. I cannot. That's part of the conc– reason I said, ultimately, I think that– you know, same-sex couples should be able to marry. That's my personal position. And, frankly, that's the position that's reflected– in the briefs that we filed– in the Supreme Court.



My hope is that– the Court looks at the evidence and– and in the California case, for example, the only reason presented for treating gays and lesbians differently was, "Well, they're gay and lesbian." There wasn't– a real rationale beyond that. In fact– you know, all the other– rights and– and– responsibilities of– a civil union were identical to marriage.



It's just you couldn't call it marriage. Well, at that point, what you're really sayin' is– "We're just gonna treat these folks differently because of who they are." And– and I do not think– that's– that's who are as Americans. And– and frankly, I think– American attitudes have evolved, just like mine have– pretty substantially and fairly quickly, and I think that's a good thing.