A civil right, or a political question

One the biggest issues facing the justices is whether they can -- or should -- issue a ruling that will effectively broaden the legal definition of marriage, long restricted to heterosexual couples. Backers of DOMA and Proposition 8 say it should be up to the public to make that decision, not the courts.

"Our most fundamental right in this country is the right to vote and the right to participate in the political process, " said Austin Nimocks of the Alliance Defending Freedom, a Christian advocacy group.

"We don't need the Supreme Court to take that right away from Americans of good faith on both sides of this issue and impose its judicial solution," Nimocks told CNN's State of the Union. "We need to leave this debate to the democratic process, which is working."

But California Attorney General Kamala Harris, who is arguing against Proposition 8, said voter-approved marriage bans "are simply unconstitutional." The Supreme Court has ruled more than a dozen times that marriage is a fundamental right, "and as it relates to a fundamental right, the court will hold that under the highest level of scrutiny."

"It is one thing to read the polls, which we have discussed, which show again that a majority of Americans are in favor of same-sex marriage," Harris said. "But it is more important to read the Constitution."

Opinion: Will gay rights infringe on religious liberty?