Mr. Lanza, who is Roman Catholic and voted against same-sex marriage in 2009, found himself on the phone recently with a priest he has known for three decades. The priest was adamant. Redefining marriage, he told Mr. Lanza, “would cause more harm than good for everyone involved,” he recalled.

Mr. Lanza objected, politely but pointedly. “My faith is an important part of my life and always has been,” he recalled telling the priest. “But nowhere can I find there being anything in my faith that is inconsistent with people being able to pursue their true selves.”

It was the first time he can recall sparring with a priest, and the experience seemed to have left him slightly rattled.

At times, he said, emotions have run so high that he has had to cut off conversations. Inside the Capitol, a man approached him and demanded to know why he would even consider legalizing same-sex weddings. “You think it’s a disease, right, being gay?” the man asked. Mr. Lanza, taken aback, said no, he did not think it was a disease and quickly walked away.

Mr. Lanza said he wished he could announce he was immovably opposed to same-sex marriage to avoid enduring the pressure from both sides. But he cannot.

“It would be a lot easier,” he said.

As he weighs how to vote, Mr. Alesi, 63, a Republican from suburban Rochester and the longtime owner of a commercial laundry company, has spoken with proud parents who want their gay children to have the same rights as their straight children. And he has heard from some opponents of same-sex marriage who are the despondent ex-husbands and ex-wives of gay people.

One of them told Mr. Alesi that a spouse’s surprise admission of homosexuality had torn apart the family. Why, this person asked, would the senator want to inflict more pain by legalizing same-sex marriage?