Stanley B. Greenberg, a Democratic pollster and chief executive of the firm Greenberg Quinlan Rosner , is the author of “America Ascendant.”

Moderate Republicans will have the last word in this dramatic presidential election year. The GOP establishment and its favored candidates view these voters as illegitimate, which is why they lost the primaries to Donald Trump. Now moderates are poised to play similarly decisive roles in the general election — by helping to elect Democrat Hillary Clinton — and in the battle for the party’s future that will follow it.

Moderates stand out starkly among the groups that make up the Republican base, for two reasons: They are disproportionately college graduates in a white, working-class party, and they are socially liberal. They have been alienated from a party that won’t accept the revolution that has occurred in American social and sexual mores and move on.

Because no candidate this cycle spoke to their issues and grievances, these voters can seem invisible. But according to polling we conducted at Democracy Corps in February, moderates make up a stunning 31 percent of the GOP base. Commentators on the ongoing GOP train wreck pay a lot of attention to the tea party, white working-class voters and rural evangelical Christians, but how much have you heard about the alienation of the moderate third of the party?

Everyone should be much more curious about why these voters did not rally to former Florida governor Jeb Bush, Sen. Marco Rubio (Fla.) or Ohio Gov. John Kasich. Well, two-thirds of GOP moderates believe abortion should be legal in “all” or “most cases,” whereas Bush, Rubio and Kasich — labeled “moderate” in the media — are firmly pro-life, differing only on the kinds of exceptions they would allow. That’s a defining issue for many moderate Republicans and helps to explain why many voted for Trump, who stood out from the pack when he allowed that Planned Parenthood “does a lot of good.”

In an interview with CBS News' "Face the Nation," Democratic front-runner Hillary Clinton explains what she means when she says Republican rival Donald Trump is a "loose cannon." (Reuters)

This is not to say that moderates have nothing in common with the rest of the Republican base. They are fiscally conservative, distrust regulation, and want lower taxes and a strong military. They would repeal Obamacare, and they want the government to get control of immigration.

But unlike much of their GOP peers, they have accepted the sexual revolution. According to our poll, nearly 90 percent say that their party should stop fighting the fact that “women and men feel free to have sex without any interest in getting married,” and half believe this strongly. Three-quarters say the party should accept legal same-sex marriage, even as an identical bloc of evangelicals and observant Catholics wants the party to fight it. And in another key area of separation, almost two-thirds accept that “scientists say 2015 was the hottest year in historical record” and “that human activity is a significant factor in climate change.” Moderates want to see the country, and their own party, make progress on equal pay for women, climate change, financial reform and long-term investment in infrastructure.

That is why GOP moderates are about to abandon their party’s nominee in large numbers, helping to elect Clinton. Our polling found that just 60 percent of GOP moderates said they would vote for Trump in a matchup with Clinton. Only 10 percent were ready to vote for Clinton, but fully 30 percent said they would vote for some other person, wouldn’t vote or weren’t sure what to do. Only 6 percent of Republicans voted for Barack Obama in 2012.

Ultimately, Clinton’s muscular views on national security, which position her to the right of Trump, may persuade some of these voters to listen to her on other issues. According to my survey, GOP moderates are moved by Clinton’s message that social changes accepted in much of the country should be set aside so we can begin “addressing our country’s problems.” Although Trump sent mixed signals about Planned Parenthood, he insists that he is pro-life and that those performing abortions should face consequences; he is frequently forced to defend comments that seem disrespectful to women who are key players and interpreters of our liberalizing United States. That allows nearly half of GOP moderates to respond positively to Clinton’s plans to invest in infrastructure to strengthen the country, to reform corporations so they no longer chase short-term profits and to help the modern working family with issues such as equal pay for women.

The Republican Party is approaching a crossroads. After the Democratic Party lost a shattering election in 1984, the civil war it went through raised new issues, brought in new voters and elected the New Democrat Bill Clinton two presidential elections later. Will the Republican Party likewise come to terms with the sexual revolution, marriage equality and climate change in the aftermath of the 2016 election? Republican leaders will be making a mistake — again — if they fail to ask: “Why did so many of our voters help elect Hillary Clinton?”