WASHINGTON – A second constitutional amendment to ban same-sex marriage is expected to fail in the Senate today, but proponents of the resolution plan to stay the congressional course and not pursue the back-door option: a constitutional convention called by state legislatures.

Supporters of the amendment say the longer they persist in their efforts to have marriage codified as solely between a man and a woman, the more likely it will eventually become enshrined in the Constitution.

“We keep seeing improvements every year,” the vice president of government affairs at the Family Research Council, Tom McClusky, said. “We certainly aren’t going to give up on Congress.”

Senate staffers and vested parties said yesterday that Senator Allard’s Marriage Protection Amendment – which defines marriage as between a man and a woman, yet leaves other legal options for same-sex couples up to state legislatures – does not have the 60 votes to bring it to a direct vote. But the amendment’s supporters said they have no plans to abandon Capitol Hill; instead, they will try to force an amendment through a little-known clause in the Constitution.

Article 5 provides two processes by which the country’s leaders may bring amendments. For the past week, the Senate has been mired in the first, requiring both houses of Congress to approve by a two-thirds majority a resolution calling for the amendment, followed by ratification by three-quarters of state legislatures.

But as with many other quirky provisions in the document, the Founding Fathers left a trapdoor for lawmakers. The legislatures of two-thirds of the states can vote to call for a nationwide constitutional convention, during which lawmakers can propose any sort of amendment, again requiring three-quarters of legislatures for it to succeed.

In the history of America, this method of amendment has never occurred. And, though they vary on perspective, even the most vocal social conservative groups – no matter how many times same-sex marriage fails in Congress – say they doubt it ever will.

“The question is why anyone would pursue that stupid idea,” a social conservative, Phyllis Schlafly, said. “I don’t see Ben Franklin, George Washington, and Thomas Jefferson around today, and I’m not encouraged by anyone who thinks they are. This would be like having the Republican and Democratic conventions in the same hall, and expose the Constitution to all sorts of factions. I don’t know of a reason to put the whole Constitution up for grabs, just for one amendment.”

Members of other groups, including Concerned Women for America and Alliance for Marriage, which put together yesterday’s pro-amendment event with President Bush, say a convention would only make matters worse for their cause.

“A one-sentence amendment will generate more support and is the fastest way to preserve traditional marriage,” Jan LaRue of Concerned Women for America said. “We’re not going to let this process with Congress become a dead issue.”

Not only would a convention potentially damage the Constitution, it would probably prove as complicated as getting a federal amendment passed, experts say.

“It hasn’t ever happened before. I suppose on the grassroots level it could, but there’s a reason it hasn’t been done in the past: fear,” a resident scholar with the American Enterprise Institute, Norman Ornstein, said.”Once you call a constitutional convention, you can’t resist the scope of things that get brought up.”

Meanwhile, in the Senate chamber, Senator Santorum, a Republican of Pennsylvania who has been a key momentum builder behind the issue’s revival, led senators weighing in on Mr. Allard’s bill yesterday. Most of the debate revolved around the 19 states that have already enacted amendments to prohibit same-sex marriage, as well as the legal challenges seeking permission for gays to marry pending in 10 states.

Some doubted the wisdom of having to outlaw same-sex marriage by changing the Constitution. “Most Americans are not yet convinced that their elected representatives or the judiciary are likely to expand decisively the definition of marriage to include same-sex couples,” Senator McCain, a Republican of Arizona, said.

Many took the floor to suggest issues the Senate should make time to consider instead of the ban. The Senate “has taken no action to fix America’s broken health care system. It has blocked passage of the Patients’ Bill of Rights. It has refused to allow a vote on raising the minimum wage,” Senator Kennedy, a Democrat of Massachusetts, said. “Rather than deal with these urgent priorities, the leadership is engaging in the politics of mass distraction by bringing up a discriminatory marriage amendment to the United States Constitution that a majority of Americans do not support.”

Staffers – including those in the offices of Mr. Santorum and Majority Leader Frist – said they expect the vote nearly to mirror the result of the Federal Marriage Amendment of 2004, which failed by a margin of 50-48 during a vote on cloture, a process to bring the issue to the floor requiring 60 votes in favor.

And though the anti-gay marriage groups largely dismiss the idea of a constitutional convention and insist they need a coalition to pursue alternative strategies, the second demise of the measure has got some thinking whether a convention might not be the best way to achieve their ends.

“We’ve thought about it, but it’s just not ours to do. There are a whole host of groups who have a secular interest in seeing a marriage amendment become a reality,” a senior trial attorney with the American Family Association, Brian Fahling, said. “We’ve almost been entirely focused on idea of getting the amendment out of Congress and signed by the president and then to the states. [The defeat of the amendment] may force us to re-examine things.”