Mark Montigny was raised by an Irish Catholic mother, went to Catholic camp as a kid and attended Catholic grammar school. He holds the church sacraments in high esteem.

He's also a state senator who supports the rights of gays to marry.

"As a Catholic, I would never vote to diminish the sanctity of the church sacrament of marriage," said Montigny, a Democrat. "As a human being, I will never vote to deny someone their equal rights. It is my belief that the only requirement of civil marriage is enduring love and respect."

A proposal to amend the state constitution to define marriage as between one man and one woman has caused particular personal conflict for many legislators in Massachusetts. The Vatican (search) has warned Roman Catholic politicians -- about 65 percent of the state's 200 legislators are Catholic -- that support of same-sex unions is "gravely immoral."

Another Catholic lawmaker, Rep. Stephen J. Buoniconti, said he would support the constitutional amendment, but only partly because of his religion.

"It's the value system that I was raised with," said Buoniconti, a Democrat. "I'm just not comfortable with gay marriage."

Massachusetts put itself at the center of the gay-marriage debate when the Supreme Judicial Court ruled in November that gays should be guaranteed the benefits of marriage. Lawmakers thought that Vermont-style civil unions (search) might suffice, but the court issued an advisory opinion last week that left no doubt: Only full-fledged gay marriage would pass constitutional muster.

That cleared the way for the nation's first legally sanctioned same-sex weddings (search) by May.

The state House and Senate were to meet in a constitutional convention Wednesday to consider the amendment to ban gay marriages and set the measure on a course to wind up before voters in 2006.

The issue has set off a frenzy of activity and lobbying at the Statehouse, where at least 101 legislators would have to vote in favor of the amendment to move it to the next step.

One last-minute proposal floated by a bipartisan group of Senate leaders would seek to amend the constitution to ban gay marriages but allow civil unions. Any couples married in the interim would be stripped of their marriage licenses, and be considered joined in a civil union.

The Catholic church has mobilized strenuous lobbying efforts in the state Legislature leading up to the constitutional convention. It also has tried to ignite the emotions of rank and file Catholics by sending letters to one million parishioners urging them to contact their lawmakers.

"The death penalty was nothing compared to the heat coming down now on this issue," said state Rep. Martin J. Walsh, a Democrat who does not support the amendment.

Archbishop Sean O'Malley (search) has appeared at rallies in opposition to gay marriage, including one this past weekend that drew thousands to Boston Common in the shadow of the Statehouse. He said the court ruling that it was unconstitutional to bar gay couples from marriage "ratifies a trend that will only harm children."

The Massachusetts Catholic Conference (search), the public policy arm of the Catholic church, said Catholic lawmakers should not be the only ones who are torn by considerations of religion and tradition. But he said Catholics, as a part of society, absolutely have a stake in the outcome.

"It has nothing to do with church and state," said Gerry D'Avolio, a lobbyist for the conference. "What we do object to is when we're no longer part of the discussion."

Rep. Carol Donovan, a Democrat, said she's concerned that her constituents, many of whom have sent her form letters provided to them by the church, are not being fully informed about the issue by religious leaders. She opposes the amendment.

"They don't understand the issue and the church is not presenting it fairly," she said.

Rep. Mark A. Howland, a Democrat, said he supports the amendment, and has tried to separate his Catholicism from his role as a lawmaker, even when his own pastor tries to weigh in.

"I tell him, 'You can tell me perhaps what to do on Sunday, if I happen to attend Mass. But Monday through Saturday I'm at the will of the voters,"' Howland said.

But for many lawmakers, the question has been among their most difficult to decide.

"I don't want to diminish the agony," Montigny said, "because it really has been a tremendous agony."