* Justices hear arguments over Defense of Marriage Act
* DOMA denies federal benefits to married same-sex couples
* Ambivalent note from Tuesday could recur in DOMA arguments
Midway into a second day of tackling the gay marriage issue, conservatives on the U.S. Supreme Court said on Wednesday they were troubled by President Barack Obama's decision in 2011 not to defend in court a ban Congress had approved.
The decision by Obama to abandon the legal defense of the 1996 Defense of Marriage Act (DOMA) called into question his willingness to defend other laws passed by Congress and challenged in court, several conservative justices said.
"It's very troubling," said Justice Anthony Kennedy.
While the criticisms may not affect how the justices eventually rule on whether the 1996 law violates U.S. equal protection rights, it showed frustration with how Obama has walked a difficult political line on gay marriage.
Obama and his attorney general, Eric Holder, said in February 2011 they would cease defending the law because they believed it to be invalid under the U.S. Constitution.
In the place of the U.S. Justice Department, Republican lawmakers have stepped in to argue for the law.
Chief Justice John Roberts pressed government lawyer Sri Srinivasan on how the government will now decide which laws to defend. "What is your test?" Roberts asked.
Justice Antonin Scalia, who served in the Justice Department in the 1970s, criticized its "new regime."
Almost two hours of oral argument will be heard by the court on DOMA. The nine justices heard arguments on Tuesday on the constitutionality of California's Proposition 8 ban on gay marriage.
In those arguments, the justices displayed a reluctance to rule broadly on the right to marry for gays and lesbians, suggesting the court may be similarly cautious about DOMA.
Rulings in both cases are expected by the end of June.
The cases come before the court as polls show growing support among Americans for gay marriage but division among the 50 states. Nine states recognize it; 30 states have constitutional amendments banning it and others are in-between.
DOMA limits the definition of marriage as between a man and a woman. It permits benefits such as Social Security survivor payments and federal tax deductions only for married, opposite-sex couples, not for legally married same-sex couples.
President Bill Clinton signed DOMA into law in 1996 after it passed Congress with only 81 of 535 lawmakers opposing it. Clinton, a Democrat, earlier this month said that times have changed since then and called for the law to be overturned.