U.S. Supreme Court Justice Elena Kagan aligned with her conservative colleagues Monday to uphold Kansas' narrow insanity defense, a ruling her fellow liberals say rips out "the core of a defense that has existed for centuries."

Justice Kagan's opinion for the 6-3 court Monday upheld the conviction of a Kansas man who in 2009 killed his ex-wife, her grandmother and his two daughters. Defendant James Kahler's attorneys argued that the state denied him due process because he wasn't allowed to use his moral incapacity — the inability to tell right from wrong — as an affirmative insanity defense to the murder charges.

Under Kansas law, the affirmative insanity defense is available only to those who, because of mental illness, didn't understand what they were doing, regardless of right or wrong. Therefore, he was only allowed to use evidence of his moral incapacity during the sentencing phase of the trial to argue against the death penalty, after he had been found guilty. The jury still chose the death penalty.

Justice Kagan's opinion is the latest in a string in which she has managed to form a majority with the court's conservatives and perhaps tempered their penchant for a ruling that would have veered even further to the right.

Writing for a majority that included all five Republican appointees on the court Monday, she said the Constitution's due process clause does not require states like Kansas to allow defendants to use their moral incapacity as an affirmative defense to criminal liability, a defense that would otherwise allow them to be found not guilty by reason of insanity.

Justice Kagan said that, under Supreme Court precedent, states are allowed to adopt their own rules about criminal liability unless a rule "offends some principle of justice so rooted in the traditions and conscience of our people as to be ranked as fundamental."

A uniform insanity test is not so fundamental, she said.

"The insanity defense sits at the juncture of medical views of mental illness and moral and legal theories of criminal culpability — two areas of conflict and change," the opinion said. "Small wonder that no particular test of insanity has developed into a constitutional baseline."

In a dissent joined by Justices Ruth Bader Ginsburg and Justice Sotomayor, Justice Stephen Breyer disagreed with that conclusion. The principle that someone's moral incapacity is an affirmative defense to criminal liability has stood for centuries.

"Kansas has not simply redefined the insanity defense," Justice Breyer said. "Rather, it has eliminated the core of a defense that has existed for centuries: that the defendant, due to mental illness, lacked the mental capacity necessary for his conduct to be considered morally blameworthy."

Citing sources from the founding era, Justice Breyer said that [t]hese fundamental principles of criminal responsibility were incorporated into American law from the early days of the Republic."

Justice Kagan's decision to team up with her conservative colleagues over the dissent of her fellow Democratic appointees is not uncommon. Just last term, Justice Kagan joined a 6-3 opinion by Justice Neil Gorsuch expanding the type of "confidential" private business information that is exempt from disclosure under the Freedom of Information Act.

Justice Breyer, who also wrote the dissent in that case on behalf of the other liberals, said the majority's ruling was "at odds" with the purpose of the law.

Considered by many court watchers more moderate than the other Democratic appointees, Justice Kagan has fought against the court's recent rightward drift by trying to get her conservative colleagues to come to the table and compromise.

In Masterpiece Cakeshop Ltd. v. Colorado Civil Rights Commission , for instance, she and Justice Breyer joined the conservatives to rule in favor of a Christian baker who refused to sell a wedding cake to a gay couple.

While the judgment came as a victory for Masterpiece owner Jack Phillips, the majority opinion disappointed some who wanted to see the court interpret the First Amendment to allow religious business owners to refuse certain expressive services to LGBT customers; the court ruled only that Colorado's civil rights agency hadn't given Phillips a fair shake in earlier proceedings, delaying the core constitutional question for another day.

Justice Kagan seemed to perform the same trick last term in the case American Legion v. American Humanist Association. There, she and Justice Breyer joined the conservatives to uphold the constitutionality of a giant Peace Cross honoring World War I dead next to a Maryland highway. But the ruling stopped short of a broader conservative victory by not getting rid of a rigid doctrine governing church-state separation dating back to the early 1970s.

The case decided Monday is Kahler v. Kansas, case number 18-6135.

--Editing by Peter Rozovsky.