The Senate in mid-July will take up a constitutional amendment banning gay marriage, forcing lawmakers to cast a tough political vote just weeks before the Democratic presidential convention in Massachusetts.

President Bush (searchhas urged Congress to move on the amendment, but sponsors acknowledge the difficulty of getting the two-thirds majority to approve it.

"We're not certain we'll be successful in this effort," Sen. John Cornyn (search), R-Texas, said at a news conference to announce that the amendment would be on the Senate floor the week of July 12.

Cornyn and the measure's chief sponsor, Sen. Wayne Allard (search), R-Col., denied they were stirring up a divisive political issue two weeks before Democrats gather in Massachusetts, the first state to recognize same-sex marriages.

"This was an issue that was thrust upon us by the Massachusetts Supreme Court," Cornyn said. "We didn't pick the battle, we didn't pick the timing."

Allard said there were at least 11 pending court cases on the issue around the country. "We must not stand still when the courts are being used to challenge and distort civilization's oldest, most venerable social institution,"he said.

Bush announced his support for the amendment last February, but recently has come under pressure from some of his conservative allies to give the issue a higher profile.

Paul M. Weyrich, national chairman of an amalgam of conservative organizations known as Coalitions for America (search), recently said that Bush needs to change the subject from Iraq to the gay marriage ban in order to be re-elected in November.

Bush responded last week by reiterating his opposition to gay marriage in remarks to the Southern Baptist Convention (search), saying that "government, by strengthening and protecting marriage, serves the interests of all."

Democratic presidential contender Sen. John Kerry has criticized the proposed amendment as an effort to drive a political wedge between Americans. He has supported civil unions and said the issue of marriage should be left to the states. Sen. John Edwards, D-N.C., a possible running mate, has expressed a similar stance.

The Christian Coalition of America (search), a strong backer of the amendment, urged people to deluge the Senate with petitions, calls, letter and faxes to ensure an early vote. "Force your senators to take a public position before voters go to the polls this fall," it said in a website message.

The House has made no decision on when it might consider the amendment, said Stuart Roy, spokesman for House Majority Leader Tom DeLay, R-Texas. "We want to pass it, we don't want to just bring it up," he said.

The Allard amendment states that "marriage in the United States shall consist only of the union of a man and a woman. Neither this Constitution, nor the constitution of any state, shall be construed to require that marriage or the legal incidents thereof be conferred upon any union other than the union of a man and a woman."

Amendments to the Constitution require approval by two-thirds majorities in the House and Senate and ratification by three-fourths of state legislatures.