It's an encouraging sign for equal rights that the U.S. Supreme Court has agreed to take on two marriage equality cases. The best-case scenario is that the high court rulings, expected sometime this summer, will reflect growing national momentum behind same-sex marriage.

Nine states and the District of Columbia have made same-sex marriage legal, and polls now show that at least 50 percent of Americans favor allowing it.

One of the cases, from New York, rightly challenges the federal Defense of Marriage Act (DOMA), a law that unfairly denies federal benefits to legally married same-sex couples. The other case, from California, could establish a constitutional right to same-sex marriage.

The DOMA case is the most clear-cut. Justices should follow the lead of the lower court and reject rules that so blatantly violate the Constitution's equal protection guarantee. The case involves an 83-year-old New York woman who challenged a $363,000 federal estate tax bill after her partner of 44 years died in 2009. The two were married in 2007 in Canada. Had she been married to a man, her tax bill would have been zero. Clearly, that kind of disparity is wrong.

Congress passed DOMA in 1996 -- well before any states had legalized gay marriage. Because of the act, a same-sex couple is denied federal benefits -- such as Social Security survivor benefits -- available to a straight couple. But now that thousands of same-sex partners are legally married under state laws, 10 consecutive lower-court rulings have rejected parts of the law.

The Justice Department now declines to defend DOMA in court, though the government continues to enforce it. The high court should put to rest those conflicting government actions once and for all.

The California case is more complex. In 2008, that state's voters approved a state constitutional ban, called Proposition 8, in response to a state Supreme Court ruling that said gays could marry.

Since then, a federal appeals court struck down the constitutional provision but did not authorize the resumption of same-sex marriages, pending appeal. The appellate court ruled that the ban was unacceptable on equal protection grounds. Still, Proposition 8 only affected legal use of the word "marriage'' in that state; same-sex couples in civil unions have access to the same benefits as straight couples.

There is precedent for federal courts to overturn discriminatory state laws. Thirteen states still had laws against sodomy when the U.S. Supreme Court said those laws represented an unconstitutional violation of privacy. Interracial marriage still was illegal in 16 states in 1967 before the high court outlawed race-based state marriage bans. And in 1954, when the court issued the landmark ruling in Brown vs. Board of Education, 17 states had formally segregated school systems.

The Star Tribune Editorial Board has long supported same-sex marriage as a matter of fairness and equal rights. There are legitimate concerns, however, that a sweeping decision on the California case that rejects all state gay-marriage bans as unconstitutional would move too far, too fast and create a national backlash. Forty-one states, including Minnesota, have yet to legalize gay marriage, and there is a danger that a sweeping ruling would be seen as judicial overreach.

A more narrow ruling rejecting Proposition 8 would allow same-sex couples to marry in California but not affect other states where support for gay marriage is evolving.