PEABODY — State Sen. Majority Leader Fred Berry, D-Peabody, says he can no longer support a constitutional amendment banning gay marriage.

"I don't think I'm a radical. I just think I'm doing the right thing," he said yesterday.

The constitutional convention, a joint meeting of the state House and Senate, approved the amendment, 105-92, in March of 2004, two months after same-sex marriage became legal under a court ruling. If the convention approves it again when it meets Sept. 14, it will go to voters in November 2006.

Berry supported the amendment last year because it was seen as a compromise at the time, he said. The measure would ban same-sex "marriage" but allow "civil unions," spelling out some protections for gay and lesbian couples.

Now he has changed his mind.

"Fifteen months into this, I have not seen one negative thing come out of gay marriage," he said. "Nothing earth-shattering has happened."

Berry said he made his decision over the summer and that attending the same-sex marriages of two friends helped him decide.

"Frankly, you realize two people are in love and they want to do the same thing everyone else does," he said. "I just wanted to be forthright and tell people I was voting this way and not surprise them."

Supporters of gay marriage say that by allowing civil unions, which grant some of the benefits of marriage, the amendment relegates homosexuals to second-class status. Opponents contend civil unions diminish the importance of the traditional marriage.

Berry's decision comes at a time when support for the constitutional amendment now under consideration is waning among both opponents and supporters of gay marriage. The shift is due in part to a citizen's petition seeking to define marriage as the "union of one man and one woman."

Opponents of same-sex marriage are now pinning their hopes on the petition, which was submitted to the Secretary of State last month. If approved, first by two successive constitutional conventions, and then by voters, it would ban future same-sex marriages but not the estimated 6,000 gay marriages since May 17, 2004. It makes no mention of rights or benefits for same-sex unions.

Gov. Mitt Romney and Catholic leaders are backing the new initiative petition, which is expected to appear on November's ballot. The final vote on the measure would come in November of 2008.

Berry said he thinks most voters would like to see lawmakers "move on" to more important business. Other people's thinking has evolved over time, just as his has, he said.

"To be totally frank, I understand the conservative aspect," he said. "I had reservations about culture shock."

But now, he said, he must vote according to his conscience.

"I don't see the evil in gay marriage," he said.