"We simply will not let activist judges redefine that definition of marriage," the Tennessee Republican said to a gathering of anti-gay marriage activists. "We will not let activist judges redefine — I would say radically redefine — what marriage is, and that is a union between a man and a woman."
Meanwhile, the New York State Attorney General said gay marriage is illegal, as the mayor of a New York town was due in court to answer criminal charges for performing gay marriages, and a second New York mayor said he would begin conducting same-sex weddings.
Elsewhere, an Oregon county issued its first same-sex marriages.
The issue of gay marriage has raced into the national spotlight in a matter of weeks. The Massachusetts high court permitted same-sex marriages, the city of San Francisco began conducting such marriages, other cities and counties followed suit, and President Bush called for a constitutional ban.
Frist's remarks opened what promises to be a divisive election-year battle on Capitol Hill.
Using the Massachusetts high court ruling permitting same-sex marriages as an impetus, Frist said that Congress should not wait until the states make a final decision on the subject.
With gay marriages already being performed in California and New York, "the wildfire will begin and in many ways it already has begun," he said. "Same sex marriage is likely to spread through all 50 states in the coming years. It is becoming increasingly clear that Congress must act."
But gay rights supporters are fighting back, framing the issue as America's next civil rights battle.
"A constitutional amendment regarding same sex marriage would write discrimination into the governing document of our nation," said Sen. Russ Feingold, D-Wis.
Feingold also called the amendment movement a "divisive political exercise in an election year, plain and simple."
"I object to the use of the constitutional amendment process for political purposes and I am sorry to say that I believe that is what I believe is going on here," Feingold said.
Most Americans are opposed to gay marriage. A CBS News poll conducted immediately after Mr. Bush endorsed a constitutional ban on gay marriage, 59 percent of Americans said they would favor an amendment to the Constitution that would "allow marriage only between a man and a woman," up slightly from 55 percent last December.
The arguments for and against a constitutional amendment fall along social and civil rights lines, and cut across others.
Pastor Daniel de Leon Sr. of Santa Ana, Calif., in testimony prepared for the Senate hearing, said marriage is for the benefit of children, not adults. He said efforts to stop gay marriage are not comparable to past opposition to biracial unions.
"Laws forbidding interracial marriage are about racism," he said. "Laws protecting traditional marriage are about children."
But in an unlikely alliance, some "limited government" conservatives, gay rights and civil rights supporters all plan to fight an amendment, even though they may not agree on the gay marriage question.
"This is not to say that conservatives such as myself necessarily favor gay marriages, but that we strongly oppose the notion of addressing this issue of social policy in our nation's governing document," said Chuck Muth, president of Citizen Outreach.
In New Paltz, N.Y., Mayor Jason West vowed to go ahead with up to two dozen same-sex weddings this weekend, despite being charged with 19 criminal counts and possibly facing jail time for marrying gay couples.
West was scheduled to be in town court Wednesday night to answer charges that he married 19 couples knowing they did not have marriage licenses, a violation of the state's domestic relations law.
"I'm incredibly disappointed," said the Green Party mayor, who added that he will plead innocent at his court hearing. "Apparently, it's a crime to uphold the constitution of New York state."
West married 25 gay couples on Friday, and has about 1,000 couples on the waiting list. More than 3,400 couples have been married in San Francisco.
The mayor of another village north of New York City, John Shields of Nyack, tells The Associated Press he'll start officiating at weddings of gay couples, possibly this week.
It was unclear if the attorney general's statement would change those plans. Eliot Spitzer said current law prohibits same-sex weddings but leaving it to the courts to decide if the law is constitutional.
"I personally would like to see the law changed, but must respect the law as it now stands,'' Spitzer said in a statement
Meanwhile, a crowd of gay couples was expected to go to the county administration building in Portland, Oregon on Wednesday after a county commissioner there said she would begin issuing marriage licenses to same-sex couples.
Multnomah County Chair Diane Linn directed the county, the state's most populous, to begin issuing such licenses after consulting with the county attorney, but without an official vote from the four other county commissioners.
A county judge said she was ready to conduct the weddings.
"Many of these couples have been waiting decades, and this is the first time they've been seen as equal under the law," said Roey Thorpe, executive director of Basic Rights Oregon.