Murphy was at Pulse on this day to show it to United States Representative Barbara Lee, a California Democrat who belongs to a quickly swelling army of party leaders who have traveled to Florida to stump for Murphy or help her raise money, reflecting the party’s identification of her contest as one that might flip a House seat from red to blue and help to erode the Republican majority.

Image Stephanie Murphy, candidate for Congress from Florida. Credit... Bill Clark/CQ Roll Call, via Associated Press

“This is a very winnable race,” Lee told me as we approached Pulse, adding that what happened there — and its exposure of the hatred that L.G.B.T. Americans still confront — is part of the equation. “I think people will see that as a defining moment and say: ‘No more. My vote is going to be for human rights.’ ”

Across many decades and hundreds of campaigns at every level of government, L.G.B.T. rights have been a point of bitter debate, often benefiting Republican politicians in conservative areas where voters pushed back at social change. In recent years, though, Democrats have increasingly sought to turn their advocacy for L.G.B.T. people into an advantage.

Public opinion polls leave no doubt that a significant majority of Americans support laws protecting L.G.B.T. people from discrimination and approve of same-sex marriage. But that doesn’t mean that they prioritize the issue and punish politicians with contrary views. The results of many elections suggest that they don’t.

I think that’s changing, and 2016 could be the proof of it. In several closely fought races around the country, candidates’ actions and comments regarding gay people have come to the fore and come to define them. Murphy’s contest against John Mica, now in his 12th term, is only one of them.