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Democrats wasted no time Tuesday lambasting President Bush for his support of a constitutional amendment to ban gay marriage (search).

"By endorsing this shameful effort to write discrimination back into the Constitution, President Bush has betrayed his campaign promise to be 'a uniter, not a divider,'" Sen. Ted Kennedy (search) of Massachusetts said on the Senate floor.

"By endorsing this shameful proposal, President Bush will go down in history as the first president to try to write bias back into the Constitution."

Bush called Tuesday for a constitutional amendment to bar gay marriage, saying the nation must defend "the most fundamental institution of civilization."

Marriage cannot be severed from its "cultural, religious and natural" roots, Bush said in his address, but left the door open for civil unions as an alternative to same-sex marriages.

The decision, which aims to shore up the president's conservative base, came after a Massachusetts court said it's unconstitutional to ban same-sex marriages and ordered the state Legislature to take action to clarify the law. Soon after the Massachusetts decision, San Francisco Mayor Gavin Newsom (search) gave city officials permission to issue marriage licenses to gay and lesbian couples.

Several groups are trying to sue to stop the gay marriage blitz, and California Attorney General Bill Lockyer (search) on Tuesday said he'll go straight to the state Supreme Court to try to resolve the divisive issue.

Democratic front-runner John Kerry (search), the Massachusetts senator hoping to run against Bush in November's presidential election, said he thinks Bush is wrong.

"All Americans should be concerned when a president who is in political trouble tries to tamper with the Constitution of the United States at the start of his re-election campaign," said Kerry, who opposes gay marriage but supports civil unions.

Bush is "looking for a wedge issue to divide the American people," Kerry said.

North Carolina Sen. John Edwards (search), Kerry's chief rival in the race, also opposes same-sex marriages but supports civil unions.

"Washington has no business playing politics with this issue. Marriage is left to the states today, and should remain with the states," Edwards said in a statement.

In an election year when every move is up for criticism, Bush critics said his support of an amendment to prevent state courts from taking the initiative and recognizing gay marriages came at a time when his poll numbers are in need of a boost and the economy is suffering.

"Beware of a president who wants to change the Constitution when his political troubles are on the rise," Michael Meehan, Kerry's senior campaign adviser, told Fox News, noting that recent polls show Bush and Kerry in almost a dead heat.

Added Edwards: "I'm not surprised that the day after he kicked off his campaign, the president is already talking about gay marriage. He can't talk about jobs, because he has no new ideas to create jobs. He can't talk about health care, because he has no new ideas to hold down costs or cover every child. He can't talk about education, because he has no new ideas to help young people pay for the soaring cost of college."

"I guess it's just a coincidence the president brought this out on the first day of his campaign," said Bruce Reed, president of the Democratic Leadership Council (search).

"It's a sign of just how desperate they are," Reed said of the Bush-Cheney re-election campaign, which kicked off Monday night with a Bush speech.

Rep. Steny Hoyer (search), D-Md., the House minority whip, said Bush's announcement is aimed at diverting attention from his "abysmal record."

Bush is "kicking off his re-election campaign by cynically exploiting a wedge issue that panders to his political base and diverts attention from his abysmal record in office," Hoyer said in a statement.

Hoyer said the pro-amendment move is designed to take focus off of the 2.3 million jobs lost during the Bush administration and the war in and reconstruction of Iraq, among other things.

"George W. Bush and Republicans will rue the day that they tried to enshrine in our celebrated Constitution."

Democratic National Committee Chairman Terry McAuliffe (search) also charged Bush with "flip-flopping" on the issue.

"Now that it is convenient to his re-election, he is using this amendment as a purely political move to turn the attention away from his failed record as president," McAuliffe said. "Since he can't run on his record of shipping jobs overseas, failing schools, and rising healthcare costs, he has to change the subject. President Bush came to the White House pledging to unite us, and is now seeking to divide the country for his own political gain"

Democratic Gov. Bill Richardson (search) added: "I believe it's very bad public policy … to tamper with the Constitution on an issue like this, which should be left up to the states … I think it's unfortunate."

Republicans, however, rushed to defend the president's move.

"A constitutional amendment defining marriage as being between one man and one woman is necessary and something I support," said Sen. Lindsey Graham (search), R-S.C. "The traditional institution of marriage is now under attack … If we don't act, it's only a matter of time until marriage as we have known it will be redefined."

Sen. John Cornyn (search), R-Texas, and chairman of the Senate Judiciary's Subcommittee on the Constitution, said Tuesday that he plans on holding his first hearing on the gay marriage issue next Wednesday.

The hearing will deal with the national implications of the Massachusetts decision. Another hearing scheduled for late March will deal with the specific language of a constitutional amendment.

GOP strategist Rick Davis denied that the White House is worried about Bush's re-election chances, saying that the president's coming out with such a big announcement as the gay-marriage amendment "is not panic at all" by the Bush-Cheney camp.

"I think this discussion of a constitutional amendment, or at least some legal limitations on gay marriage ... is a legitimate topic for the nation."