Feinstein's legislation to rescind the Defense of Marriage Act gets it first hearing Wednesday in Washington.
Feinstein says Americans are more open to gay marriage than they were 15 years ago. She said she doesn't know how long it will take to change the current law, but she believes Congress is taking an important first step.
Researchers estimate that between 50,000 and 80,000 same-sex couples have legally married in the handful of states that have approved those marriages.
Speaking at the National Press Club, Feinstein made it clear that she's determined to repeal the Defense of Marriage Act. The 15-year-old law also known as DOMA defines marriage as between a man and a woman. Feinstein says the law is unfair to gay couples.
"Because of DOMA, these couples cannot take advantage of federal protections available to every other married couple in the country," she said.
New York recently approved a law allowing gay marriage.
California's voter-approved ban on same-sex unions was declared unconstitutional last year by a federal judge. That decision is being appealed. Among other things, DOMA prohibits gay couples from filing joint income taxes and receiving spousal benefits.
"Many people are impacted precisely when they're the most vulnerable, after the death of a spouse or when their spouse is seriously ill," Feinstein said. "This discrimination is just that: plain and simple discrimination."
On Wednesday, the Senate Judiciary Committee will hold its first hearing on Feinstein's bill.
"This legislation will uphold the principle that the federal government should not deny gay and lesbian couples the same rights and legal protections as straight couples," said White House Press Secretary Jay Carney.
SaveCalifornia, a group that opposes same-sex marriage, issued a statement: "Even if the natural and beautiful institution of marriage is attacked by Dianne Feinstein and the Senate Democrats, this bad idea will be stopped, thankfully, by the Republican house."
Feinstein said if her bill to repeal the law isn't approved this session, the effort will continue until, "the battle is won."