BOSTON —  Democrat John Kerry (search) and running mate John Edwards (search) may end up not having an opportunity to vote on a constitutional amendment to ban gay marriage (search), a polarizing issue in the presidential campaign.

President Bush (search) has called for the amendment and top Republicans are trying to get one passed in the Senate. But opponents of the measure are trying to block the amendment from ever coming to a direct vote, and the only vote likely to occur now is a procedural one scheduled for Wednesday aimed at forcing the Senate to act on the amendment.

Kerry spokeswoman Stephanie Cutter said Kerry and Edwards would be in the Senate to vote against the amendment if it came up, but they will not be there to vote on the procedural measure.

Republicans, who had already conceded they lacked the two-thirds majority — or 67 votes — needed to advance a constitutional amendment, would have to get 60 votes to go to a final vote on the issue itself.

On Wednesday, Kerry is scheduled to be in his hometown of Boston preparing for the Democratic National Convention being held there later this month. Edwards, the North Carolina senator, begins his first solo campaign swing since being named to the ticket.

While Kerry and Edwards oppose gay marriage, they argue that it is an issue that should be left to the states to decide. Both senators support civil unions, which would give gay couples all the legal rights of married couples without letting them wed.