President Bush, flanked by conservative supporters and religious leaders, will hold a White House press conference this morning to throw his authority behind a constitutional amendment that would outlaw same-sex marriage.

The event will launch the Senate debate on the resolution introduced by Senator Allard, a Republican of Colorado, defining marriage in America as a union between a man and a woman, with a vote expected on Wednesday.

Although there is little chance that the change to the Constitution will achieve the required majority of votes demanded by the Constitution to become law, the issue is seen as a means of shoring up conservative support for the administration ahead of the November elections.

President Bush has already attached his support to the measure, devoting his weekly radio address to restate his belief that “ages of experience have taught us that the commitment of a husband and a wife to love and to serve one another promotes the welfare of children and the stability of society. The constitutional amendment that the Senate will consider … would fully protect marriage from being redefined.”

Senators who support the amendment say that with some states continuing to ban same-sex marriage and several lawsuits proceeding through the courts demanding that same-sex unions be declared legal, the time has once again come to raise the issue, despite its 50-48 Senate defeat in 2004.

“The leader feels the courts have forced the Senate’s hand,” said Carolyn Weyforth, a spokeswoman for the Senate majority leader, Bill Frist, a Republican of Tennessee, who is a proponent of the bill.

But many in Washington see the revival of the amendment as political pandering in the summer before an election season as the president’s record low approval rating threatens to jeopardize the chances of Republicans being elected in the midterm elections in November.

“Republicans like Bill Frist are using issues like marriage as political wedges to distract people from their failed leadership and misplaced priorities,” the chairman of the Democratic National Committee, Howard Dean, said in a statement. “Because Americans have lost confidence in their ability to solve challenges like Iraq, immigration, health care, gas prices, or homeland security, Republicans in Washington are ripping a page from the Karl Rove playbook by scapegoating gay and lesbian families for partisan gain.”

Few on either side of the fence expect the Allard bill to pass. Worded less strongly than 2004’s Federal Marriage Amendment, the new amendment would prohibit states from recognizing same-sex marriages, but leave state legislatures free to define legal arrangements short of marriage for gay couples.

To gain the two-thirds majority it needs to get through the Senate, the measure faces significant obstacles, experts say. During the 2004 elections, the Republicans gained four Senate seats, but five Republican members have been vociferous in their opposition to such a constitutional amendment, including Senators McCain, a Republican of Arizona, Snowe, a Republican of Maine, and Chafee, a Republican of Rhode Island. Only one Democrat, Senator Nelson of Nebraska, has so far said he will vote for it.

“They got 48 votes the last time and they’re likely to get 52 the next time, but essentially everyone in the Senate the last time will probably vote the same way,” said Joe Solmonese, president of the Human Rights Campaign, a gay rights group. Even if the amendment went to the Senate, then the House, it would still require the ratification of the 38 state legislatures.

The Senate debate coincides with Gay Pride Week, which has encouraged gay rights activists to converge on Washington to ensure a Constitutional amendment doesn’t dampen their celebrations.

Timed to coincide with Mr. Bush’s White House press conference, several groups, including members of the clergy, will hold a press conference at the National Press Club to announce their opposition to the measure. And as Senators begin their debating, their staffers can expect 250,000 postcards from Human Rights Campaign members to arrive on the Hill.

While the president’s radio address on Saturday surprised some, many in Washington expected the same-sex marriage amendment to return from its 2004 grave at some point this year.

But with the first lady, Laura Bush, publicly declaring to Fox News that she thinks the issue should be treated tactfully and carefully and should not become a matter of political campaigning, and with Vice President Cheney’s daughter Mary highlighting going on television to promote her memoir, which includes her coming out as gay, during which she repeatedly stresses her father’s past opposition to the amendment, calling it “writing discrimination into the Constitution” during a book tour, it was expected Mr. Bush would remain on the sidelines of the debate.

In recent weeks, however, many social conservative groups, including the Family Research Council, have been pushing Mr. Bush to take another stand, hoping to bolster the faltering support among conservative Republicans comprising his base. Following the president’s mid-May Oval Office address on immigration, Tony Perkins, the head of the FRC, sent an e-mail to members questioning the president’s priorities, according to the Washington Times.

“It’s not that we are demanding this, but when the first lady is disparaging the issue, and when the vice-president lets stand unrebutted Mary Cheney’s claims, we think some demonstration of presidential leadership is warranted – and overdue,” he wrote.

Mr. Solmonese said Mr. Bush’s change of direction made good political sense. “As we continue to see the diminishing base of support for this administration, we had to begin to wonder what things he was going to pull out in an effort to mobilize.

“He’s lost the middle and losing everyone else, so he’s beginning to see he’s got to really think about how to motivate and activate the base.”