“The more that the advocates for this are able to tell Republicans that you can do this and live, then the greater success they’re going to have,” said Robert J. Bellafiore, a communications consultant and former press secretary for Gov. George E. Pataki, a Republican. “But it’s hard to convince somebody to take their first bungee jump if all they see at the bottom of the jump is a bunch of splattered bodies.”

Ms. Marchione, 58, a resident of Halfmoon, had one previous moment in the statewide spotlight: In 2007, she was among the most vocal opponents of an ill-fated plan by Gov. Eliot Spitzer, a Democrat, to issue driver’s licenses to illegal immigrants. In the campaign this year, she often brings up the issue of same-sex marriage — one of her campaign mailers quoted Mr. McDonald’s “shove it” comment; another showed side-by-side photographs of Mr. McDonald and President Obama, compared their positions on same-sex marriage and derided the senator as a “Barack Obama Republican.”

In an interview, Ms. Marchione described Mr. McDonald as “one of the most liberal senators in the Republican caucus.”

“People come back so many times to me and say, ‘I believe marriage is between a man and a woman,’ ” Ms. Marchione said. Still, she said, if elected, she would not seek to repeal the Marriage Equality Act. “This issue has passed,” she said, given the large Democratic majority in the State Assembly and the presence of a governor who supports same-sex marriage.

Mr. McDonald, who was also once a state assemblyman, said he would not regret his marriage vote even if it ended his political career. But he lamented a modern political culture that he described as filled with vitriol and devoid of compassion.

And while he said he did not want to be seen as a martyr for gay rights — “I’m not looking for any more of my 15 minutes of fame; I’ve had enough. I just want to do my job” — he expressed frustration that some Republicans seemed more eager to make the primary about same-sex marriage than, say, chronic unemployment.